MEDICAL    SCHOOL 


Gift  of  H.W.  Sheldon 


3TATE  MEPICAL  LTBRAKY 


HEREDITY 

IN   RELATION   TO   EVOLUTION 
AND  ANIMAL  BREEDING 


HEREDITY 

IN  RELATION  TO  EVOLUTION 
AND  ANIMAL  BREEDING 


BY 
WILLIAM   E.  CASTLE 

PROFESSOR    OF    ZOOLOGY,    HARVARD    UNIVERSITY 


NEW  YORK  AND  LONDON 
D.  APPLETON  AND  COMPANY 


COPYRIGHT,  1911,  BY 
D.  APPLETON  AND  COMPANY 


Printed  in  the  United  States  of  America 


PREFACE 

THIS  little  book  is  based  on  a  course  of  eight 
lectures  delivered  in  November  and  December, 
1910,  before  the  Lowell  Institute,  Boston,  as  well 
as  on  a  course  of  five  lectures  delivered  before 
the  Graduate  School  of  Agriculture  held  under 
the  auspices  of  the  Association  of  Agricultural 
Colleges  and  Experiment  Stations  at  Ames, 
Iowa,  in  July,  1910.  The  hope  is  entertained 
that  it  may  be  of  service  to  students  and  that  it 
will  also  interest  the  general  reader. 

The  writer  wishes  to  express  his  gratitude  to 
the  Carnegie  Institution  of  Washington  for  per- 
mission, in  its  preparation,  to  draw  freely  upon 
published  and  unpublished  material  derived  from 
investigations  aided  by  the  Institution. 

Acknowledgment  is  also  due  to  the  following 
persons,  or  to  their  publishers,  for  permission  to 
use  figures  from  their  publications,  as  indicated 
in  the  text :  Prof.  E.  B.  Wilson  and  The  Mac- 
millan  Co.,  Prof.  H.  S.  Jennings  and  The  Ameri- 
can Naturalist,  Dr.  W.  B.  Kirkham  and  The 
American  Book  Co. 

W.  E.  CASTLE 

JUNE,  1911 


CONTENTS 

PAGE 

INTRODUCTION.  —  GENETICS,  A  NEW  SCIENCE   ...         1 

CHAPTER 

I.  —  THE  DUALITY  OF  INHERITANCE 6 

II.  —  GERM-PLASM    AND    BODY,    THEIR   MUTUAL 

INDEPENDENCE  27 

III.  —  MENDEL'S  LAW  OF  HEREDITY 33 

IV.  —  THE  DETERMINATION  OF  DOMINANCE; 

HETEROZYGOUS    CHARACTERS    AND    THEIR 
FIXATION;  ATAVISM  OR  REVISION.    ...       52 

V.  —  EVOLUTION   OF    NEW  RACES   BY  Loss   OR 

GAIN  OF  CHARACTERS 72 

VI.  —  EVOLUTION  OF  NEW  RACES  BY  VARIATIONS 

IN  THE  POTENCY  OF  CHARACTERS  ....       87 

VII.  —  CAN  MENDELIAN  UNIT-CHARACTERS  BE 

MODIFIED  BY  SELECTION? 106 

VIII.  —  MENDELIAN   INHERITANCE  WITHOUT  DOMI- 
NANCE, " BLENDING"  INHERITANCE  .    .    .     128 

IX.  —  THE  EFFECTS  OF  INBREEDING 143 

X.  —  HEREDITY  AND  SEX 153 

INDEX  183 


LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

1.  —  Egg  and  sperm  of  the  sea-urchin,  Toxopneustes  9 

2. —  Fertilization  of  the  egg  of  .A^m's 12 

3.  —  Egg  of  a  mouse  previous  to  maturation  Facing  14 

4.  —  Maturation   and   fertilization  of   the    egg  of 

a  mouse Facing       14 

5.  —  Diagrams    showing    the    essential    facts    of 

chromosome  reduction  in  the  development 

of  the  sperm-cells 17 

6. — An  ordinary  fern 21 

7.  —  The  prothallus  of  a  fern 23 

8.  —  Diagram  showing  the  chromosome  number  in 

the    spermatogenesis   of   ordinary   animals 

and  of  the  wasp 24 

9.  —  Diagram  showing  the  relation  of  the  body  to 

the  germ-cells  in  heredity 29 

10.  —  A  young,  black  guinea-pig Facing  30 

11.  —  An  albino  female  guinea-pig Facing  30 

12.  —  An  albino  male  guinea-pig Facing  30 

13.  —  Pictures  of  three  living  guinea-pigs  and  of  the 

preserved  skins  of  three  others .    .    .  Facing       32 

14.  —  A  black,  female   guinea-pig,  and  her  young 

Facing       34 

15.  —  An  albino  male  guinea-pig Facing       34 

16.  —  Two  of  the  grown-up  young  of  a  black  and  of 

an  albino  guinea-pig Facing       34 


IX 


xii  LIST  OF   ILLUSTRATIONS 

FIG.  PAGE 

47.  —  Diagrams  to  show  the  number  and  size  of  the 

classes  of  individuals  to  be  expected  from 
a  cross  involving  Mendelian  segregation 
without  dominance 135 

48.  —  Photographs  to  show  variation  in  ear  length 

of  two  varieties  of  maize,  of  their  Fx  off- 
spring, and  of  their  F2  offspring  .  .Facing  138 

49.  —  Diagram  of   sex-determination   in   partheno- 

genesis   162 

50.  —  Diagram  of  sex  determination  when  the  female 

is  homozygous,  the  male  heterozygous     ..  .     167 

51.  —  Diagram  of  sex-determination  when  the  female 

is  heterozygous,  the  male  homozygous    .    .     170 

52.  —  Diagram  of  sex-limited  inheritance  when  the 

female  is  a  heterozygote 173 

53.  —  Diagram  of  sex-limited  inheritance  when  the 

female  is  a  homozygote,  as  in  the  red-eyed 
Drosophila 175 


HEREDITY 

INTRODUCTION 

GENETICS,   A   NEW    SCIENCE 

THE  theory  of  organic  evolution  has  prob- 
ably influenced  more  fields  of  human 
activity  and  influenced  them  more  pro- 
foundly than  has  any  other  philosophic  deduc- 
tion of  ancient  or  modern  times.  By  this  theory 
philosophy,  religion,  and  science  have  been  rev- 
olutionized, while  in  the  practical  arts  of  educa- 
tion and  agriculture,  twin  foundation  stones  of 
the  state,  man  has  been  forced  to  adopt  new 
methods  of  procedure  or  to  justify  the  old  ones 
in  the  light  of  a  new  principle. 

The  evolutionary  idea  has  forced  man  to  con- 
sider the  probable  future  of  his  own  race  on 
earth  and  to  take  measures  to  control  that  fu- 
ture, a  matter  he  had  previously  left  largely  to 
fate.  With  a  realization  of  the  fact  that  or- 


HEEEDITY 

ganisms  change  from  age  to  age  and  that  he 
himself  is  one  of  these  changing  organisms  man 
has  attained  not  only  a  new  ground  for  humility 
of  spirit  but  also  a  new  ground  for  optimism  and 
for  belief  in  his  own  supreme  importance,  since 
the  forces  which  control  his  destiny  have  been 
placed  largely  in  his  own  hands. 

The  existence  of  civilized  man  rests  ultimately 
on  his  ability  to  produce  from  the  earth  in  suf- 
ficient abundance  cultivated  plants  and  domes- 
ticated animals.  City  populations  are  apt  to 
forget  this  fundamental  fact  and  to  regard  with 
indifference  bordering  at  times  on  scorn  agri- 
cultural districts  and  their  workers.  But  let  the 
steady  stream  of  supplies  coming  from  the  land 
to  any  large  city  be  interrupted  for  only  a  few 
days  by  war,  floods,  a  railroad  strike,  or  any 
similar  occurrence,  and  this  sentiment  vanishes 
instantly.  Man  to  live  must  have  food,  and 
food  comes  chiefly  from  the  land. 

A  knowledge  of  how  to  produce  useful  animals 
and  plants  is  therefore  of  prime  importance. 
Civilization  had  its  beginning  in  the  attainment 
of  such  knowledge  and  is  limited  by  it  at  the 
present  day.  If,  therefore,  this  knowledge  can 

2 


GENETICS,   A    NEW    SCIENCE 

be  increased,  civilization  may  be  advanced  in  a 
very  direct  and  practical  way.  Before  Darwin 
the  practices  of  animal  and  plant  breeders  were 
largely  empirical,  based  on  unreasoned  past  ex- 
perience, just  as  was  in  antiquity  the  practice 
of  metallurgy.  Good  plows  and  good  swords 
were  made  long  before  a  scientific  knowledge 
of  the  metals  was  attained,  but  without  that  sci- 
entific knowledge  the  wonderful  industrial  de- 
velopment of  this  present  age  of  steel  would 
have  been  quite  impossible.  In  a  similar  way, 
if  not  in  like  measure,  we  may  reasonably  hope 
for  an  advance  in  the  productiveness  of  animal 
and  plant  breeding  when  the  scientific  principles 
which  underlie  these  basic  arts  are  better  under- 
stood. Two  practical  problems  present  them- 
selves to  the  breeder :  (1)  how  to  make  best  use 
of  existing  breeds,  and  (2)  how  to  create  new 
and  improved  breeds  better  adapted  to  the  con- 
ditions of  present-day  agriculture.  We  shall 
concern  ourselves  with  the  second  of  these  only. 
The  production  of  new  and  improved  breeds 
of  animals  and  plants  is  historically  a  matter 
about  which  we  know  scarcely  more  than  about 

the  production  of  new  species  in  nature.    Selec- 

3 


HEREDITY 

tion  has  been  undoubtedly  the  efficient  cause  o 
change  in  both  cases,  but  how  and  why  applie< 
and  to  what  sort  of  material  is  as  uncertaii 
in  one  case  as  in  the  other.  The  few  grea 
men  who  have  succeeded  in  producing  by  thei 
individual  efforts  a  new  and  more  useful  typ 
of  animal  or  plant  have  worked  largely  b; 
empirical  methods.  They  have  produced  ; 
desired  result  but  by  methods  which  neithe 
they  nor  any  one  else  fully  understood  o 
could  adequately  explain.  So  there  exists  a 
yet  no  true  science  of  breeding  but  only  ; 
highly  developed  art  which  was  practiced  a 
successfully  by  the  ancient  Egyptians,  th 
Saracens,  and  the  Eomans  as  by  us.  Th 
present,  however,  is  an  age  of  science;  w 
are  not  satisfied  with  rule-of-thumb  methods 
we  want  to  know  the  why  as  well  as  the  hoi 
of  our  practical  operations.  Only  such  knowl 
edge  of  the  reasons  for  methods  empirically 
successful  can  enable  us  to  drop  out  of  ou 
practice  all  superfluous  steps  and  roundabou 
methods  and  to  proceed  straight  to  the  marl 
in  the  most  direct  way.  The  industrial  his 
tory  of  the  last  century  is  full  of  instances  ii 

4 


GENETICS,    A    NEW    SCIENCE 

which  a  knowledge  of  causes  in  relation  to 
processes,  i.  e.  a  scientific  knowledge,  has 
shortened  and  improved  practice  in  quite  un- 
expected ways.  So  we  may  not  doubt  the  ulti- 
mate value  in  practice  of  a  science  of  breeding, 
if  such  a  science  can  be  created. 

A  beginning  has  been  made  during  the  last 
ten  years,  starting  with  the  rediscovery  of 
MendePs  law  of  heredity  in  1900.  This  book 
will  be  concerned  largely  with  the  operations 
of  that  law. 


CHAPTER   I 

THE   DUALITY   OF   INHERITANCE 

A  the  outset  we  may  with  profit  inquire 
what  is  meant  by  heredity.     When  a 
child   resembles   a   parent   or   grand- 
parent in  some  striking  particular,  we  say  it 
inherits    such-and-such    a    characteristic   from 
the   parent  or  grandparent  in  question.     By 
heredity,  then,  we  mean  organic  resemblance 
based  on  descent. 

Eesemblances  due  to  heredity  may  exist  even 
between  individuals  not  related  as  ancestor  and 
descendant,  as  for  example  between  uncle  and 
nephew.  Here  the  resemblance  rests  on  the 
fact  that  uncle  and  nephew  are  both  descended 
from  a  common  ancestor,  and  they  resemble 
each  other  simply  because  they  have  both  in- 
herited the  same  characteristic  from  that  an- 
cestor. This  form  of  inheritance  is  sometimes 
spoken  of  as  collateral  in  distinction  from  direct 

6 


THE    DUALITY    OF    INHEKITANCE 

inheritance.  In  all  cases  alike  community  of 
descent  is  the  basis  of  resemblances  which  can 
be  ascribed  to  heredity,  whether  direct  or  col- 
lateral. Mother  and  child,  no  less  than  uncle 
and  nephew,  resemble  each  other  because  they 
have  received  a  common  inheritance  from  a 
common  ancestor. 

Three  biological  facts  of  fundamental  im- 
portance to  a  right  understanding  of  heredity 
were  known  imperfectly  or  not  at  all  in  the 
time  of  Darwin  and  Mendel.  These  are  (1) 
the  fertilization  of  the  egg,  (2)  the  maturation 
of  the  egg,  which  must  precede  its  fertilization, 
and  (3)  the  non-inheritance  of  "  acquired  ' 
characters.  These  we  may  consider  in  order. 

Every  new  organism  is  derived  from  a  pre- 
existing organism,  so  far  as  our  present  ex- 
perience goes.  It  may  not  have  been  so  al- 
ways. Indeed,  on  the  evolution  theory,  we 
must  suppose  that  living  matter  originally 
arose  from  lifeless,  inorganic  matter.  But  if 
it  did,  this  may  have  occurred,  and  probably 
did  occur,  under  physical  conditions  quite 
different  from  those  now  existing.  At  the 
present  time  the  most  exhaustive  researches 

7 


HEREDITY 

fail  to  reveal  the  occurrence  of  spontaneous 
generation,  that  is,  the  origin  of  living  beings 
other  than  from  pre-existing  living  beings. 

In  asexual  methods  of  reproduction  a  new 
individual  arises  out  of  a  detached  portion  of 
the  parent  individual.  Such  methods  of  origin 
are  varied  and  interesting,  but  do  not  concern 
us  at  present.  In  all  the  higher  animals  and 
plants  a  new  individual  arises,  by  what  we  call 
a  sexual  process,  from  the  union  of  two  minute 
bodies  called  the  reproductive  cells.  They  are 
an  egg-cell  furnished  by  the  mother  and  a 
sperm-cell  furnished  by  the  father. 

There  is  a  great  difference  in  size  between 
egg  and  sperm.  The  egg  is  many  thousand 
times  greater  in  bulk,  as  seen  in  Fig.  1,  for 
example,  yet  the  influence  of  each  in  heredity 
appears  to  be  equal  to  that  of  the  other.  This 
fact  shows  unmistakably  that  the  bulk  of  the 
reproductive  cell  is  not  significant  in  heredity. 
A  large  part  of  the  relatively  huge  egg  can 
have  no  part  in  heredity.  It  serves  merely  as 
food  for  the  new  organism,  furnishing  it  with 
building  material  until  such  a  time  as  it  can 
begin  to  secure  food  for  itself.  The  essential 

8 


THE    DUALITY    OF    INHERITANCE 

material,  so  far  as  heredity  is  concerned,  is 
evidently  found  in  egg  and  sperm  alike.  It 
is  plainly  small  in  amount  and  possibly  con- 
sists merely  in  ferment-like  bodies  which,  ini- 


FIG.  1.  —  Egg  and  sperm  (s)  of  the  sea-urchin,  Toxopneustes, 
both  shown  at  the  same  enlargement.     (After  Wilson.) 

tiate  certain  metabolic  processes  in  a  suitable 
medium  represented  by  the  bulk  of  the  egg. 
The  amount  of  a  ferment  used  in  starting  a 
chemical  change  bears  no  relation,  as  is  well 
known,  to  the  amount  of  the  chemical  change 
which  it  can  bring  about  in  a  suitable  medium. 

9 


HEREDITY 

The  equal  share  of  egg  and  sperm  in  deter- 
mining the  character  of  offspring  is  well  shown 
in  the  following  experiment.  An  albino  guinea- 
pig  is  one  which  lacks  in  large  measure  the 
ability  to  form  black  pigment.  Apparently  it 
does  not  possess  some  ingredient  or  agency 
necessary  for  the  production  of  pigment.  Now, 
if  an  albino  male  guinea-pig,  such  as  is  shown 
in  Fig.  15,  be  mated  with  a  black  female  guinea- 
pig  of  pure  race,  such  as  is  shown  in  Fig.  14, 
young  are  produced  all  of  which  are  black,  like 
the  mother,  none  being  albinos,  like  the  father. 
Fig.  16  shows  black  offspring  produced  in  this 
way.  Exactly  the  same  result  is  obtained  from 
the  reverse  cross,  that  is,  from  mating  an  al- 
bino mother  with  a  black  sire.  It  makes  no 
difference,  then,  whether  the  black  parent  be 
mother  or  father,  its  blackness  regularly  domi- 
nates over  the  whiteness  of  the  albino  parent, 
so  that  only  black  offspring  result.  This  fact, 
which  has  been  repeatedly  confirmed,  shows  that 
the  black  character  is  transmitted  as  readily 
through  the  agency  of  the  minute  sperm-cell 
as  through  the  enormously  greater  egg-cell. 

Let  Us  now  consider  what  happens  when  egg 
10 


THE    DUALITY    OF   INHEEITANCE 

and  sperm  unite,  in  what  we  call  the  fertiliza- 
tion of  the  egg.  The  egg  is  a  rounded  body 
incapable  of  motion,  but  the  sperm  is  a  minute 
thread-like  body  which  moves  like  a  tadpole 
by  vibrations  of  its  tail.  In  the  case  of  most 
animals  which  live  in  the  water,  egg  and  sperm- 
cells  are  discharged  into  the  water  and  there 
unite  and  develop  into  a  new  individual,  but 
in  the  case  of  most  land  animals  this  union  takes 
place  within  the  body  of  the  mother.  We  may 
consider  an  illustration  of  either  sort. 

The  fertilization  of  the  egg  of  a  marine 
worm,  Nereis,  is  shown  in  Fig.  2.  The  thread- 
like sperm  penetrates  into  the  egg.  Its  en- 
larged head-end  forms  there  a  small  nuclear 
body,  which  increases  in  size  until  it  equals 
that  of  the  egg-nucleus,  with  which  it  then  fuses. 
The  egg  next  begins  to  divide  up  to  form  the 
different  parts  of  a  new  worm-embryo.  To 
each  of  these  parts  the  nuclear  material  of  egg 
and  sperm  is  distributed  equally.  Since  this 
development  takes  place  wholly  outside  the 
body  of  either  parent  it  is  necessary  that  the 
egg  contain  enough  food  to  last  until  the  young 
worm  can  feed  itself.  This  food  material  is 

11 


HEREDITY 


FIG.  2.  —  Fertilization  of  the  egg  of  Nereis. 
A.  The  sperm  has  entered  the  egg  and  is  forming  a  minute 
nucleus  at  a*.  The  egg-nucleus  is  breaking  up  preparatory 
to  the  first  maturation  division.  B.  The  egg-nucleus  is 
undergoing  the  first  maturation  division.  Notice  the  con- 
spicuous rod-like  chromosomes  separating  into  two  groups. 
The  sperm-nucleus  ( $ )  is  now  larger  and  lies  deeper  in  the 
egg.  C.  A  small  polar-cell  has  been  formed  above  by  the 
first  maturation  division  of  the  egg.  A  second  division  is 
in  progress  at  the  same  point.  The  sperm-nucleus  is  now 
deep  in  the  egg  and  is  preceded  by  a  double  radiation  (am- 
phiaster).  D.  Two  polar-cells  are  fully  formed.  The  ma- 
tured egg-nucleus  is  now  fusing  with  the  sperm-nucleus. 
An  amphiaster  indicates  that  division  of  the  egg  will  soon 
take  place.  (After  Wilson.) 

12 


THE    DUALITY    OF    INHEBITANCE 

represented  in  part  by  the  conspicuous  oil- 
drops  seen  in  the  egg  (the  heavy  circles  in 
Fig.  2). 

The  egg  of  a  mouse  needs  no  such  store  of 
nourishment,  since  in  common  with  the  young 
of  other  mammals  the  mouse-embryo  nourishes 
itself  by  osmosis  from  the  body  fluids  of  the 
mother.  The  mouse-egg  is  accordingly  smaller. 
Stages  in  its  fertilization  are  shown  in  Fig.  4. 
In  A  the  sperm  has  already  entered  the  egg. 
Remnants  of  its  thread-like  tail  may  still  be 
seen  there.  Nearby  is  seen  a  nuclear  body 
derived  from  the  sperm-head.  Opposite  is 
seen  the  nuclear  body  furnished  by  the  egg 
itself.  The  two  nuclear  bodies  fuse  and  their 
united  substance  is  then  distributed  to  all 
parts  of  the  embryo-mouse,  just  as  happens  in 
the  development  of  the  worm,  Nereis. 

There  are  reasons  for  thinking  that  the 
nuclear  material  is  especially  important  in  re- 
lation to  heredity  and  that  the  equal  share  of 
the  two  parents  in  contributing  it  to  the  em- 
bryo is  not  without  significance,  for  inheritance, 
as  we  have  seen,  is  from  both  parents  in  equal 
measure.  In  cases  where  the  inheritance  from 

13 


HEEEDITY 

each  parent  is  different  it  can  be  shown 
that  the  offspring  possess  two  inherited  possi- 
bilities, though  they  may  show  but  one.  Thus 
in  the  case  of  a  black  guinea-pig,  one  of  whose 
parents  was  white,  the  other  black,  it  can  be 
shown  that  the  animal  transmits  both  qualities 
(black  and  white)  which  it  received  from  its 
respective  parents,  and  transmits  them  in  equal 
measure.  For,  if  the  cross-bred  black  animal 
be  mated  with  a  white  one,  half  the  offspring 
are  black  and  half  of  them  white.  The  cross- 
bred black  animal  inherited  black  from  one 
parent,  white  from  the  other.  It  showed  only 
the  former,  but  on  forming  its  reproductive 
cells  it  transmitted  black  to  half  of  these,  white 
to  the  other  half.  Hence  the  cross-bred  black 
individual  was  a  duality,  containing  two  possi- 
bilities, black  and  white,  but  its  reproductive 
cells  were  again  single,  containing  either  black 
or  white,  but  not  both. 

Now  it  has  been  shown  in  recent  years  that 
the  nuclear  material  in  the  reproductive  cells 
behaves  exactly  as  do  black  and  white  in  the 
cross  just  described.  This  nuclear  material 
becomes  doubled  in  amount  at  fertilization, 

14 


FIG.  3.  —  Egg  of  a 
mouse  previous  to 
maturation.  (After 
Kirkham.) 


FIG.  4.  —  Maturation  and  fertilization  of  the  egg  of  a  mouse. 
A.  The  first  maturation  division  in  progress.  B,  The  first 
polar-cell  fully  formed  ;  the  second  maturation  division 
in  progress.  C.  The  second  maturation  division  com- 
pleted ;  the  second  polar-cell  is  the  smaller  one  ;  near  it, 
in  the  egg,  is  the  egg-nucleus,  and  at  the  left  is  the  sperm- 
nucleus.  D.  A  view  similar  to  the  last,  but  showing  only 
one  polar-cell,  the  second;  note  its  twelve  distinct 
chromosomes ;  near  the  sperm-nucleus  in  the  egg,  at 
the  left,  is  seen  the  thread-like  remains  of  the  sperm-tail. 
(After  Kirkham.) 


THE    DUALITY    OF   INHERITANCE 

equal  contributions  being  made  by  egg  and 
sperm.  This  double  condition  persists  through- 
out the  life  of  the  new  individual  in  all  its 
parts  and  tissues.  But  if  the  individual  forms 
eggs  or  sperm,  these,  before  they  can  function 
in  the  production  of  a  new  individual,  must 
undergo  reduction  to  the  single  condition. 

This  reduction  process  is  called  maturation; 
it  is  well  illustrated  in  the  case  of  the  mouse- 
egg,  whose  fertilization  has  already  been  de- 
scribed. The  large  nucleus  of  the  egg-cell,  as 
it  leaves  the  ovary,  is  either  broken  up  or  about 
to  break  up  preparatory  to  a  cell-division.  The 
most  conspicuous  of  the  nuclear  constituents 
are  some  dense,  heavily  staining  bodies  called 
chromosomes,  about  twenty-four  in  number. 
In  Fig.  3  each  of  these  is  split  in  two,  prepara- 
tory to  the  first  maturation  division.  The  egg 
now  divides  twice,  both  times  very  unequally 
(Fig.  4),  forming  thus  two  smaller  cells  called 
polar  cells,  or  polar  bodies.  They  take  no  part 
in  the  formation  of  the  embryo.  The  chromo- 
somes left  in  the  egg  after  these  two  divisions 
are  only  about  half  as  numerous  as  before,  or 
about  twelve  in  number.  These  form  the  chro- 

15 


HEREDITY 

matin  contribution  of  the  egg  to  the  production 
of  a  new  individual.  It  is  possible  that  other 
cell  constituents  undergo  a  similar  reduction 
by  half  during  maturation,  but  of  this  we  have 
no  present  knowledge. 

The  known  fact  of  chromosome  reduction, 
of  course,  favors  the  current  interpretation 
that  the  chromosomes  are  bearers  of  heredity, 
though  it  by  no  means  proves  the  correctness 
of  that  interpretation.  In  the  egg  of  Nereis, 
as  well  as  in  that  of  the  mouse,  two  matura- 
tion divisions  precede  the  fertilization  of  the 
egg.  See  Fig.  2.  In  B  the  first  maturation 
division  is  in  progress;  in  C  the  second  is  in 
progress;  and  in  D  both  polar  cells  are  fully 
formed,  while  egg  and  sperm  nuclei  are  unit- 
ing. Similar  processes  occur  in  eggs  gener- 
ally, prior  to  their  fertilization. 

Like  changes  occur  also  in  the  development 
of  the  sperm-cells.  In  Fig.  5  the  original  or 
unreduced  condition  of  the  chromosomes  in  a 
cell  of  the  male  sexual  gland  is  shown  (at  A)  as 
one  of  four  chromosomes  to  a  cell.  After  a  series 
of  changes  involving  as  in  the  maturation  of  the 
egg  two  cell-divisions,  we  find  (at  H)  that  the 

16 


THE    DUALITY    OF    INHERITANCE 

products  of  the  original  cell  contain  in  each  case 
two   chromosomes,   half  the   original  number. 


FIG.  5.  —  Diagrams  showing  the  essential  facts  of  chromosome 
reduction  in  the  development  of  the  sperm-cells.  (After 
Wilson.) 

These  chromosomes  make  up  the  bulk  of  the 
head  of  the  sperm  which  forms  from  each  of 


HEREDITY 

these  cells,  its  tail  being  derived  from  other 
portions  of  the  cell. 

It  follows  that  not  only  eggs  but  also  sperms, 
prior  to  their  union  in  fertilization  have  passed 
into  a  reduced  or  single  state  as  regards  their 
chromatin  constituents,  whereas  the  fertilized 
egg,  and  the  organism  which  develops  from  it, 
is  in  a  double  condition.  It  will  be  convenient 
to  refer  to  the  single  condition  as  the  N  condi- 
tion, the  double  as  the  2  N  condition. 

From  a  wholly  different  source  we  have 
evidence  strongly  confirmatory  of  the  conclu- 
sion that  the  fertilized  egg  contains  a  double 
dose  of  the  essential  nuclear  material.  By  arti- 
ficial means  it  has  been  found  possible  to  cause 
the  development  of  an  unfertilized  egg.  The 
means  employed  may  be  of  several  different 
sorts,  such  as  stimulation  with  acids,  alkalies, 
or  solutions  of  altered  density.  In  such  ways 
the  development  has  been  brought  about  of  the 
eggs  of  sea-urchins,  star-fishes,  worms,  and  mol- 
lusks,  which  normally  require  fertilization  to 
make  them  develop. 

The  sea-urchin  egg  has  been  made  to  develop 
more  successfully  than  any  other.  This  has 

18 


THE    DUALITY    OF    INHERITANCE 

occurred  even  after  the  egg  had  undergone 
maturation,  being  reduced  to  the  N  condition. 
From  the  development  of  such  reduced  but 
unfertilized  eggs  fully  normal  sea-urchins  have 
been  obtained  which  even  contain  developed 
sexual  glands.  On  the  other  hand  it  has  been 
found  possible  to  break  the  egg  into  fragments 
by  shaking  it,  or  cutting  it  into  bits  with  fine 
knives  or  scissors.  It  has  also  been  found 
possible  to  bring  about  the  development  of 
an  egg  fragment  so  obtained,  —  a  fragment 
which  contained  no  egg  nucleus.  This  result 
has  been  attained  by  allowing  a  sperm  to  enter 
it  and  form  there  a  nuclear  body.  No  adult 
organism  has  yet  been  reared  from  such  a 
fertilized  egg-fragment,  but  so  far  as  the  de- 
velopment has  been  followed  it  progressed 
normally. 

There  can  accordingly  be  no  doubt  that  the 
nuclear  material  of  a  sperm-cell  has  all  the 
capabilities  of  that  of  an  egg-cell  and  can  in- 
deed replace  it  in  development.  Accordingly, 
when,  as  in  normal  fertilization,  both  an  egg 
nucleus  and  a  sperm  nucleus  are  present  in 
the  cell,  a  double  dose  of  the  necessary  nuclear 

19 


HEREDITY 

material  is  supplied.  The  second  or  extra  dose 
is,  however,  not  superfluous.  It  probably  adds 
to  the  vigor  of  the  organism  produced,  and  in 
some  cases  at  least,  materially  affects  its  form. 
For  many  animals  and  plants  exist  in  two 
different  conditions,  in  one  of  which  the  nu- 
clear components  are  simple,  N,  while  in  the 
other  they  are  double,  2  N.  Thus  in  bees, 
rotifers,  and  small  Crustacea  the  egg  may 
under  certain  conditions  develop  without  being 
fertilized.  If  the  egg  develops  before  matura- 
tion is  complete,  that  is  in  the  2  N  condition, 
the  animal  produced  is  a  female,  like  the 
mother  which  produced  the  egg.  But  if  the 
egg  undergoes  reduction  to  the  N  condition 
before  beginning  its  development,  then  it  pro- 
duces a  male  individual,  an  organism,  so  far 
as  reproduction  is  concerned,  of  lower  meta- 
bolic activity. 

In  many  plants,  too,  individuals  of  N  and 
of  2  N  constitution  occur,  which  differ  markedly 
in  appearance.  Thus  the  ordinary  fern-plant 
is  a  2  N  individual,  but  it  never  produces  2  N 
offspring.  Fig.  6  shows  an  ordinary  fern- 
plant,  which  produces  spores  on  the  under 

20 


THE    DUALITY    OP    INHERITANCE 


FIG.  6.  —  An  ordinary  fern,  which  reproduces  by  asexual  spores. 
The  fern  is  shown  reduced  in  size  at  382;  a  portion  of  a 
frond  seen  from  below  and  slightly  enlarged,  at  383;  a 
cross-section  of  the  same  more  highly  magnified,  at  384. 
Notice  in  384  the  sporangia,  and  in  385  one  of  these  dis- 
charging spores.  (After  Wossidlo,  from  Coulter  Barnes 
and  Cowle's  Textbook  of  Botany.) 


3 


21 


HEEEDITY 

surface  of  its  fronds.  Each  of  those  spores 
is  a  reproductive  cell  which,  like  the  mature 
eggs  and  sperm  of  animals,  is  in  a  reduced 
nuclear  condition  (N).  These  spores  germi- 
nate, however,  without  uniting  in  pairs  and 
form  a  plant  different  from  the  parent,  just 
as  the  mature  egg  of  a  bee,  if  unfertilized, 
develops  into  an  individual  different  from  the 
parent,  in  that  case  a  male.  The  plant  which 
develops  from  the  spore  of  a  fern  is  small  and 
inconspicuous  and  is  known  as  a  prothallus. 
See  Fig.  7.  It  produces  sexual  cells  (eggs  and 
sperm)  which,  uniting  in  pairs,  form  fern-plants, 
2  N  individuals.  Thus  there  is  a  constant  alter- 
nation of  generations,  fern-plants  (2  N),  which 
produce  prothalli  (N),  and  then  these  produce 
again  fern-plants  (2  N). 

The  fact  is  worthy  of  note  that  in  an  animal 
or  plant  which  is  in  the  single  or  N  condition, 
there  occurs  no  chromatin  reduction  at  the 
formation  of  reproductive  cells.  Its  cells  are 
already  in  the  single  condition,  and  they 
probably  cannot  be  further  reduced  without 
destroying  the  organism.  The  2  N  fern-plant 
forms  reproductive  cells,  its  spores,  which  are 

22 


THE    DUALITY    OF   INHEBITANCE 

in  the  reduced  condition,  N,  and  these  germi- 
nate into  the  prothallus,  which  accordingly  is 


FIG.  7.  —  The  prothallus  of  a  fern,  which  reproduces  by  sexual 
cells,  eggs  and  sperm.  The  eggs  are  borne  in  the  sac-like 
"archegonia,"  just  below  the  notch  hi  the  figure.  They, 
like  the  sperm -forming  "antheridia,"  lie  on  the  under  sur- 
face of  the  flattened  prothallus  which  is  here  viewed  from 
below.  Notice  the  root-hairs  or  rhizoids  by  which  the 
plant  feeds.  Highly  magnified.  (After  Coulter,  Barnes, 
and  Cowles.) 

N  throughout.  But  when  the  prothallus  forms 
reproductive  cells,  no  reduction  occurs.  Its 
egg-cells  and  its  sperm-cells  in  common  with 

23 


HEKEDITY 

all  other  cells  of  the  prothallus  are  already 
in  the  reduced  condition  without  any  matura- 
tion divisions.  The  result  of  their  union  in 
pairs,  at  fertilization,  is  the  formation  of  2  N 
combinations  that  germinate  into  fern-plants. 
Similarly  in  the  case  of  a  male  animal  which 


FIG.  8.  —  Diagram  showing  the  chromosome  number  in  the 
spermatogenesis  of  ordinary  animals  (upper  line)  and  of  the 
wasp  (lower  line). 

has  developed  from  a  reduced  but  unfertilized 
egg,  no  reduction  occurs  at  the  formation  of 
its  sperm-cells.  In  an  ordinary  male  animal, 
one  which  is  in  the  double  or  2  N  state,  the 
development  of  the  sperms  is  attended  by  re- 
duction to  the  N  condition.  In  this  process 
there  occur  two  cell-divisions  producing  from 
each  initial  cell  four  sperms.  See  Fig.  5,  and 


THE    DUALITY    OF   INHERITANCE 

Fig.  8,  upper  line.  But  in  the  male  wasp, 
whose  cells  are  in  the  N  condition  at  the  be- 
ginning, one  of  these  divisions  is  so  far  sup- 
pressed that  the  resulting  cell  products  are  of 
very  unequal  size,  and  the  smaller  one  contains 
no  nuclear  material.  The  other  then  gives  rise 
to  two  sperm-cells,  each  possessing  the  origi- 
nal N  nuclear  condition,  while  the  small  non- 
nucleated  cell  degenerates.  See  Fig.  8,  lower 
line. 

In  conclusion,  I  wish  to  introduce  two  tech- 
nical terms,  which  it  will  be  convenient  for  us 
to  use  in  subsequent  discussions.  These  are 
gamete  and  zygote.  A  reproductive  cell  (either 
egg  or  sperm)  which  is  in  the  reduced  condi- 
tion (N)  ready  for  union  in  fertilization  is 
called  a  gamete.  The  result  of  fertilization  is 
a  zygote,  a  joining  together  of  two  cells  each 
in  the  N  condition.  The  result  is  a  new  or- 
ganism, at  first  a  single  cell,  in  the  2  N 
condition. 


HEREDITY 


BIBLIOGRAPHY 

CASTLE,  W.  E. 

1903.    "The  Heredity  of  Sex."    Bull.  Mus.  Comp.  Zool- 
ogy, 40,  pp.  189-218. 
DELAGE,  Y. 

1898.  "Embryons  sans  noyau  maternel."    Compte  rendu, 
Academic  des  sciences,  Paris,  127,  pp.  528-531. 

1909.  "Le  sexe  chez  les  Oursins  issus  de  parthe- 
nogenese  experimentale."  Compte  rendus,  Academic  des 
sciences,  Paris,  148,  pp.  453-455. 

KlRKHAM,    W.    B. 

1907.  Maturation  of  the  Egg  of  the  White  Mouse." 
Trans.  Conn.  Acad.  of  Arts  and  Sciences,  13,  pp.  65-87. 

LOEB,  J. 

1899.  "On  the  Nature  of  the  Process  of  Fertilization  and 
the  Artificial  Production  of  Normal  Larvae  (Plutei)  from 
the  Unfertilized  Eggs  of  the  Sea-urchin."    Amer.  Journ. 
of  Physiol,  3,  pp.  135-138. 

LOTSY,   J.    P. 

1905.     "Die   X-Generation   und   die  2   X-Generation." 

Biologisches  Centralblatt,  25,  pp.  97-117. 
MEVES,  F.,  und  DUESBERG,  J. 

1908.  "Die   Spermatozytenteilungen  bei   der   Hornisse 
(Vespa  crabo  L.)."    Arch.  f.  mik.  Anat.  u.  Entwick.,  71, 
pp.  571-587. 

WILSON,  E.  B. 

1896.  "The  Cell  in  Development  and  Inheritance,"  370 
pp.,  illustrated.  The  Macmillan  Co.,  New  York. 


CHAPTEE   II 

GEKM-PLASM    AND   BODY,    THEIK    MUTUAL. 
INDEPENDENCE 

IN  the  last  chapter  we  discussed  two  bio- 
logical principles  which,  if  clearly  grasped, 
greatly  simplify  an  understanding  of  the 
process  of  heredity.    These  are  as  follows: 

(1)  A  sexually  produced  individual  arises 
from  the  union  of  two  reproductive  cells   (or 
gametes),   each  of  which  contains,   so  far  as 
heredity  is  concerned,  a  full  material  equip- 
ment for  the  production  of  a  new  individual. 
Accordingly,  the  newly  produced  individual  is 
two-fold  or  duplex  as  concerns  the  material 
basis  of  heredity. 

(2)  If  the  new  individual  becomes  adult  and 
forms  gametes,  the  production  of  these  will  be 
attended   by   a   reduction   to    the    simplex   or 
single  condition  as  regards  the  material  basis 
of  heredity. 

27 


HEREDITY 

To  these  two  principles  we  may  now  add 
a  third,  viz.:  —  (3)  The  individual  consists 
of  two  distinct  parts:  first,  its  body  destined 
to  die  and  disintegrate  after  a  certain  length 
of  time;  and,  secondly,  the  germ-cells  con- 
tained within  that  body,  capable  of  indefinite 
existence  in  a  suitable  medium. 

The  fertilized  egg  or  zygote  begins  its  in- 
dependent existence  by  dividing  into  a  number 
of  cells.  These  become  specialized  to  form 
the  various  parts  and  tissues  of  the  body, 
muscle,  bone,  nerve,  etc.,  and  by  becoming  thus 
specialized  they  lose  the  power  to  produce  any- 
thing but  their  own  particular  kind  of  special- 
ized tissue;  they  cannot  reproduce  the  whole. 
This  function  is  retained  only  by  certain  un- 
differentiated  cells  found  in  the  reproductive 
glands  and  known  as  germ-cells.  They  are 
direct  lineal  descendants  of  the  fertilized  egg 
itself.  If  they  are  destroyed  the  individual 
loses  the  power  of  reproduction  altogether. 

External  influences  which  act  upon  the  body 
may  of  course  modify  it  profoundly,  but  such 
modifications  are  not  transmitted  through  the 
gametes,  because  the  gametes  are  not  derived 

28 


GEftM-PLASM    AND    BODY 

from  body-cells,  but  from  germ-cells.  This 
relationship  first  pointed  out  by  Weismann 
may  be  expressed  in  a  diagram,  as  in  Fig.  9. 
Only  such  environmental  influences  as  directly 
alter  the  character  of  the  germ-cells  will  in 
any  way  influence  the  character  of  subsequent 
generations  of  individuals  derived  from  those 


FIG.  9.  —  Diagram  showing  the  relation  of  the  body  (S)  to  the 
germ-cells  (G)  hi  heredity.     (After  Wilson.) 

germ-cells.  Body  (or  somatic)  influences  are 
not  inherited.  This  knowledge  we  owe  largely 
to  Weismann,  who  showed  experimentally  that 
mutilations  are  not  inherited.  The  tails  of 
mice  were  cut  off  for  twenty  generations  in 
succession,  but  without  effect  upon  the  char- 
acter of  the  race.  Weismann  also  pointed  out 
the  total  lack  of  evidence  for  the  then  current 
belief  that  characters  acquired  by  the  body 

are  inherited.    The  correctness  of  his  view  that 

• 

body  and  germ-cells  are  physiologically  distinct 

39 


HEREDITY 

is  indicated  by  the  results  obtained  when  germ- 
cells  are  transplanted  from  one  individual  to 
another. 

Heape  showed  some  twenty  years  ago  that 
if  the  fertilized  egg  of  a  rabbit  of  one  variety 
(for  example  an  angora,  i.  e.  a  long-haired, 
white  animal)  be  removed  from  the  oviduct  of 
its  mother  previous  to  its  attachment  to  the 
uterine  wall,  and  be  then  transferred  to  the 
oviduct  of  a  rabbit  of  a  different  variety  (for 
example  a  Belgian  hare,  which  is  short-haired 
and  gray),  the  egg  will  develop  normally  in 
the  strange  body  and  will  produce  an  individual 
with  all  the  characteristics  of  the  real  (an- 
gora) mother  unmodified  by  those  of  the  foster 
mother  (the  Belgian  hare).  Young  thus  ob- 
tained by  Heape  were  both  long-haired  and 
albinos,  like  the  angora  mother.  To  this  ex- 
periment the  objection  might  be  offered  that 
the  transplanted  egg  was  already  full-grown 
and  fertilized  when  the  transfer  was  made,  and 
that  therefore  no  modification  need  be  expected, 
but  if  the  egg  were  transferred  at  an  earlier 
stage  the  result  might  have  been  different.  In 
answer  to  such  a  possible  objection  the  follow- 

30 


FIG.  10.  —  A  young,  black  guinea-pig,  about  three 
weeks  old.  Ovaries  taken  from  an  animal  like 
this  were  transplanted  into  the  albino  shown 
below. 

FIG.  11. — An  albino  female  guinea-pig.  Its 
ovaries  were  removed,  and  in  their  place  were 
introduced  ovaries  from  a  young,  black  guinea- 
pig,  like  that  one  shown  in  Fig.  10. 

FIG.  12.  —  An  albino  male  guinea-pig,  with  which 
was  mated  the  albino  shown  in  Fig.  11. 


GEEM-PLASM   AND   BODY 

ing  experiment  performed  by  Dr.  John  C.  Phil- 
lips and  myself  may  be  cited. 

A  female  albino  guinea-pig  (Fig.  11)  just  at- 
taining sexual  maturity  was  by  an  operation 
deprived  of  its  ovaries,  and  instead  of  the  re- 
moved ovaries  there  were  introduced  into  her 
body  the  ovaries  of  a  young  black  female 
guinea-pig  (Fig.  10),  not  yet  sexually  mature, 
aged  about  three  weeks.  The  grafted  animal 
was  now  mated  with  a  male  albino  guinea-pig 
(Fig.  12).  From  numerous  experiments  with 
albino  guinea-pigs  it  may  be  stated  emphati- 
cally that  normal  albinos  mated  together,  with- 
out exception,  produce  only  albino  young,  and 
the  presumption  is  strong,  therefore,  that  had 
this  female  not  been  operated  upon  she  would 
have  done  the  same.  She  produced,  however, 
by  the  albino  male  three  litters  of  young, 
which  together  consisted  of  six  individuals,  all 
black.  (See  Fig.  13.)  The  first  litter  of  young 
was  produced  about  six  months  after  the  oper- 
ation, the  last  one  about  a  year.  The  trans- 
planted ovarian  tissue  must  have  remained  in 
its  new  environment  therefore  from  four  to 
ten  months  before  the  eggs  attained  full  growth 

31 


HEEEDITY 

and  were  discharged,  ample  time,  it  would  seem, 
for  the  influence  of  a  foreign  body  upon  the 
inheritance  to  show  itself  were  such  influence 
possible. 

In  the  light  of  the  three  principles  now 
stated,  viz.  (1)  the  duplex  condition  of  the 
zygote,  (2)  the  simplex  condition  of  the 
gametes,  and  (3)  the  distinctness  of  body  and 
germ-cells,  we  may  proceed  to  discuss  the 
greatest  single  discovery  ever  made  in  the 
field  of  heredity,  —  Mendel's  law. 


BIBLIOGKAPHY 

CASTLE,  W.  E.,  and  PHILLIPS,  JOHN  C. 

1911.     "On  Germinal  Transplantation  in  Vertebrates." 
Carnegie    Institution    of   Washington,    Publication    No. 
144,  26  pp.,  2  pi. 
HEAPE,  W. 

1890.     "Preliminary  Note  on  the  Transplantation  and 
Growth  of  Mammalian  Ova  within  a  Uterine  Foster- 
mother.'7    Proc.  Roy.  Soc.,  48,  pp.  457-458. 
1897.    "Further  Note,"  etc.    Id.  62,  pp.  178-183. 
WEISMANN,  A. 

1893.     "The  Germ-Plasm."    Translation  by  Parker  and 
Romfeldt.    Chas.  Scribner's  Sons,  New  York. 


FIG.  13.  —  Pictures  of  three  living  guinea-pigs  (A,  B,  (T), 
and  of  the  preserved  skins  of  three  others  (D,  E,  F); 
all  of  which  were  produced  by  the  pair  of  albinos  shown 
in  Figs.  11  and  12. 


CHAPTER   III 


MENDEL'S  LAW  OF  HEREDITY 


GEGOR  JOHANN  MENDEL  was  a 
teacher  of  the  physical  and  natural 
sciences  in  a  monastic  school  at  Briinn, 
Austria,  in  the  second  half  of  the  last  cen- 
tury. He  was,  therefore,  a  contemporary  of 
Darwin,  but  unknown  to  him  as  to  nearly 
all  the  great  naturalists  of  the  period.  Al- 
though not  famous  in  his  lifetime,  it  is  clear 
to  us  that  he  possessed  an  analytical  mind 
of  the  first  order,  which  enabled  him  to  plan 
and  carry  through  successfully  the  most  origi- 
nal and  instructive  series  of  studies  in  hered- 
ity ever  executed.  The  material  which  he  used 
was  simple.  It  consisted  of  garden-peas,  which 
he  raised  in  the  garden  of  the  monastery. 
The  conclusions  which  he  reached  were  like- 
wise simple.  He  summed  them  up,  the  results 
of  eight  years  of  arduous  work,  in  a  brief 
paper  published  in  the  proceedings  of  the  local 

33 


HEREDITY 

scientific  society.  There  they  remained  un- 
heeded for  thirty-four  years,  until  their  author 
had  long  been  dead.  Meantime  biological  sci- 
ence had  made  steady  progress.  It  reached 
the  position  Mendel  had  attained  in  advance 
of  his  time,  and  Menders  law  was  rediscov- 
ered simultaneously  in  1900  by  De  Vries  in 
Holland,  by  Correns  in  Germany,  and  by 
Tschermark  in  Austria.  It  gratifies  our  sense 
of  poetic  justice  that  to-day  the  rediscovered 
law  bears  the  name,  not  of  any  one  or  of  all  of 
its  brilliant  rediscoverers,  but  of  the  all-but- 
forgotten  Mendel. 

The  essential  features  of  this  law  can  best 
be  explained  in  connection  with  some  illustra- 
tions, which  I  choose  for  convenience  from  my 
own  experiments.  If  a  black  guinea-pig  of 
pure  race  (Fig.  14)  be  mated  with  a  white  one 
(Fig.  15),  the  offspring  will,  as  explained  on 
page  10,  all  be  black;  none  will  be  white. 
To  use  Mendel's  terminology,  the  black  char- 
acter dominates  in  the  cross,  while  white 
recedes  from  view.  The  black  character  is, 
therefore,  called  the  dominant  character; 
white,  the  recessive  character. 

34 


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8 


ft 
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n5  »o  cd^  N^ 


MENDEL'S    LAW    OF   HEREDITY 

But,  if  now  two  of  the  cross-bred  black  in- 
dividuals (Fig.  16)  be  mated  with  each  other, 
the  recessive  white  character  reappears  on  the 
average  in  one  in  four  of  the  offspring  (Fig. 


FIG.  18.  —  Diagram  to  explain  the  result  shown  in  Fig.  17. 

17).  Its  reappearance  in  that  particular  pro- 
portion of  the  offspring  may  be  explained  as 
follows  (see  Fig.  18) :  The  gametes  which 
united  in  the  original  cross  were,  one  black, 
the  other  white  in  character.  Both  characters 

35 


HEEEDITY 

were  then  asociated  together  in  the  offspring; 
but  black  from  its  nature  dominated,  because 
white  in  this  case  is  due  merely  to  the  lack  of 
some  constituent  supplied  by  the  black  gamete. 
But  when  the  cross-bred  black  individuals  on 
becoming  adult  form  gametes,  the  black  and 
the  white  characters  separate  from  each  other 
and  pass  into  different  cells,  since,  as  we  have 
seen,  gametes  are  simplex.  Accordingly,  the 
eggs  formed  by  a  female  cross-bred  black  are 
half  of  them  black,  half  of  them  white  in  char- 
acter, and  the  same  is  true  of  the  sperms 
formed  by  a  male  cross-bred  black.  The  com- 
binations of  egg  and  sperm  which  would  natu- 
rally be  produced  in  fertilization  are  accord- 
ingly 1BB:2BW:1WW,  or  three  combina- 
tions containing  black  to  one  containing  only 
white,  which  is  the  ratio  of  black  to  white  off- 
spring observed  in  the  experiment. 

Now  the  white  individual  may  be  expected 
to  transmit  only  the  white  character,  never  the 
black,  because  it  does  not  contain  that  char- 
acter. Experiment  shows  this  to  be  true. 
White  guinea-pigs  mated  with  each  other  pro- 
duce only  white  offspring.  But  the  black  in- 

36 


bO 

£ 


MENDEL'S    LAW    OF    HEREDITY 

dividuals  of  this  generation  are  of  two  sorts, 
-  B  B  and  B  W  in  character.  The  B  B  indi- 
vidual is  pure,  so  far  as  its  breeding  capacity 
is  concerned.  It  can  form  only  black  (B) 
gametes.  But  the  B  W  individuals  may  be 
expected  to  breed  exactly  like  the  cross-bred 
blacks  of  the  previous  generation,  forming 
gametes,  half  of  which  will  carry  B,  half  W. 
Experiment  justifies  both  these  expectations. 
The  test  may  readily  be  made  by  mating  the 
black  animals  one  by  one  with  white  ones. 
The  pure  (or  B  B)  black  individual  will  pro- 
duce only  black  offspring,  whereas  those  not 
pure,  but  B  W  in  character,  will  produce  off- 
spring half  of  which  on  the  average  will  be 
black,  the  other  half  white.  These  two  kinds 
of  dominant  individuals  obtained  in  the  second 
generation  from  a  cross  we  may  for  conven- 
ience call  homozygous  and  heterozygous,  fol- 
lowing the  convenient  terminology  of  Bateson. 
A  homozygous  individual  is  one  in  which  like 
characters  are  joined  together,  as  B  with  B; 
a  heterozygous  individual  is  one  in  which  unlike 
characters  are  joined  together,  as  B  with  W. 
It  goes  without  saying  that  recessive  individ- 
4  37 


HEREDITY 

uals  are  always  homozygous,  as  WW  for  ex- 
ample. For  they  do  not  contain  the  dominant 
character,  otherwise  they  would  show  it. 

It  will  be  observed  that  in  the  cross  of  black 
with  white  guinea-pigs  black  and  white  behave 
as  units  distinct  and  indestructible,  which  may 
meet  in  fertilization  but  separate  again  at  the 
formation  of  gametes.  Mendel's  law  as  illus- 
trated in  this  cross  includes  three  principles: 
(1)  The  existence  of  unit-characters,  (2)  domi- 
nance,  in  cases  where  the  parents  differ  in  a 
unit-character,  and  (3)  segregation  of  the  units 
contributed  by  the  respective  parents,  this  seg- 
regation being  found  among  the  gametes  formed 
by  the  offspring. 

The  principles  of  dominance  and  segregation 
apply  to  the  inheritance  of  many  characteristics 
in  animals  and  plants.  Thus  in  guinea-pigs  a 
rough  or  resetted  coat  (Figs.  23  and  24)  is  domi- 
nant over  the  ordinary  smooth  coat.  If  a  pure 
rough  individual  is  crossed  with  a  smooth  one, 
all  the  offspring  are  rough ;  but  in  the  next  gen- 
eration smooth  coat  reappears  in  one  fourth  of 
the  offspring,  as  a  rule.  Again,  in  guinea-pigs 
and  rabbits  a  long  or  angora  condition  of  the 

38 


FIG.  20.  —  Radiograph  of  a  hand  similar  to  those  shown  in 
Fig.  19.  Notice  the  short,  two-jointed  fingers.  (After 
Farabee.) 


MENDEL'S    LAW   OF   HEEEDITY 

fur  is  recessive  in  crosses  with  normal  short 
hair.  All  the  immediate  offspring  of  such  a 
cross  are  short-haired,  but  in  the  next  genera- 
tion long  hair  reappears  in  approximately  one 
fourth  of  the  offspring. 

In  cattle,  the  polled  or  hornless  condition  is 
dominant  over  the  normal  horned  condition; 
in  man,  two- jointed  fingers  and  toes  (Figs.  19 
and  20)  are  dominant  over  normal  three- jointed 
ones.  This  is  clear  from  an  interesting  pedi- 
gree given  by  Farabee  of  the  inheritance  of 
the  abnormality  in  a  Pennsylvania  family  (see 
Fig.  21).  In  no  case  was  an  abnormal  mem- 
ber of  the  family  known  to  have  married  any 
but  an  unrelated  normal  individual.  It  will 
be  seen  that  approximately  half  the  offspring 
throughout  the  four  generations  of  offspring 
shown  in  the  table  were  of  the  abnormal  sort, 
-  short-bodied  and  with  short  fingers  and  toes. 

In  each  of  the  cases  thus  far  considered  a 
single  unit-character  is  concerned.  Crosses  in 
such  cases  involve  no  necessary  change  in  the 
race,  but  only  the  continuance  within  it  of  two 
sharply  alternative  conditions.  But  the  result 
is  quite  different  when  parents  are  crossed 

39 


HEREDITY 


Cw. 

0-, 


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IQ-+  l«^ 

H 


*°    I 

m 


o- 


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«0 
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CH- 
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40 


FIG.  22.  —  A  smooth,  dark  guinea-pig. 

FIG.  23.  —  A  rough,  white  guinea-pig. 

FIG.  24. —  A  dark,  rough  guinea-pig.  The  new 
combination  of  characters  obtained  when 
animals  are  mated  like  those  shown  in 
Figs.  22  and  23. 


MENDEL'S    LAW   OF   HEEEDITY 

which  differ  simultaneously  in  two  or  more 
independent  unit-characters.  Crossing- them»be- 
comes  an  active  agency  for  the  production  of 
new  varieties. 

In  discussing  the  crosses  now  to  be  described 
it  will  be  convenient  to  refer  to  the  various 
generations  in  more  precise  terms,  as  Bateson 
has  done.  The  generation  of  the  animals  orig- 
inally crossed  will  be  called  the  parental  gen- 
eration (P) ;  the  subsequent  generations  will 
be  called  filial  generations,  viz.  the  first  filial 
generation  (F±),  second  filial  (F2),  and  so  on. 

When  guinea-pigs  are  crossed  of  pure  races 
which  differ  simultaneously  in  two  unit-charac- 
ters, the  F±  offspring  are  all  alike,  but  the  F2 
offspring  are  of  four  sorts.  Thus,  when  a 
smooth  dark  animal  (Fig.  22)  is  crossed  with 
a  rough  white  one  (Fig.  23)  the  Ft  offspring 
are  all  rough  and  dark  (Fig.  24),  manifesting 
the  two  dominant  unit-characters,  —  dark  coat 
derived  from  one  parent,  rough  coat  derived 
from  the  other.  But  the  F2  offspring  are  of 
four  sorts,  viz.  (1)  smooth  and  dark,  like  one 
grandparent,  (2)  rough  and  white,  like  the 
other  grandparent,  (3)  rough  and  dark,  like 

41 


HEEEDITY 

the  F±  generation,  and  (4)  smooth  and  white, 
a  new  variety  (Fig.  25).  It  will  be  seen  that 
the  pigmentation  of  the  coat  has  no  relation 
to  its  smoothness.  The  dark  animals  are  either 
rough  or  smooth,  and  so  are  the  white  ones. 
Pigmentation  of  the  coat  is  evidently  a  unit- 
character  independent  of  hair-direction,  and  as 
new  combinations  of  these  two  units  the  cross 
has  produced  two  new  varieties,  —  the  rough 
dark  and  the  smooth  white. 

Again,  hair-length  is  a  unit-character  inde- 
pendent of  hair-color.  For  if  a  short-haired 
dark  animal  (either  self  or  spotted,  Fig.  26) 
be  crossed  with  a  long-haired  albino  (Fig.  27), 
the  F±  offspring  are  all  short-haired  and  dark 
(Fig.  28) ;  but  the  F2  offspring  are  of  four 
sorts,  viz.  (1)  dark  and  short-haired,  like  one 
grandparent,  (2)  white  and  long-haired,  like 
the  other,  (3)  dark  and  long-haired,  a  new  com- 
bination (Fig.  29),  and  (4)  white  and  short- 
haired,  a  second  new  combination  (compare 
Fig.  25). 

Now  the  four  sorts  of  individuals  obtained 
from  such  a  cross  as  this  will  not  be  equally 
numerous.  As  we  noticed  in  connection  with 

42 


FIG.  25 


FIG.  2  9. 


FIG.  25.  —  A  smooth,  white  guinea-pig.  A  second  new  combination  of 
characters,  but  obtained  first  among  the  grandchildren  of  such  an- 
imals as  are  shown  in  Figs.  22  and  23. 

FIG.  26.  —  A  short-haired,  pigmented  guinea-pig.  ("Dutch-marked" 
with  white.) 

FIG.  27.  —  A  long-haired,  albino  guinea-pig. 

FIG.  28.  —  Offspring  produced  by  animals  of  the  sorts  shown  hi  Figs.  26 
and  27.  One  shows  the  "  Dutch-marked  "  pattern  as  a  belt  of 
pale  yellow ;  the  other  does  not.  Both  are  short-haired  and 
pigmented  (not  albinos). 

FIG.  29.  —  A  long-haired,  pigmented  guinea-pig,  "  Dutch-marked " 
with  white.  Its  parents  were  like  the  animals  shown  in  Fig.  28 ; 


sVi/vnrn     in     TTir 


9A     anrl     97 


MENDEL'S    LAW    OF    HEREDITY 

the  black-white  cross,  dominant  individuals  are 
to  the  corresponding  recessives  as  three  to  one. 
Therefore,  we  shall  expect  the  short-haired  in- 


Gameles 


Gametes 


FIG.  30.  —  Diagram  to  explain  the  result  of  a  cross  between  the 
sorts  of  guinea-pigs  shown  in  Figs.  26  and  27.  L  stands 
for  long  hair,  S  for  short  hair,  D  for  dark  hair,  and  W  for 
white  hair.  Dominant  characters  are  indicated  by  heavy 
type. 

dividuals  in  F2  to  be  three  times  as  numerous 
as  the  long-haired  ones,  and  dark  ones  to  be 
three  times  as  numerous  as  white  ones.  Fur- 
ther, individuals  which  are  both  short-haired 

43 


HEREDITY 

and  dark  should  be  3  X  3  or  9  times  as  nu- 
merous as  those  which  are  not.  The  ex- 
pected proportions  of  the  four  classes  of  F2 
offspring  are  accordingly  9:3:3:1,  a  propor- 
tion which  is  closely  approximated  in  actual 
experience.  The  Mendelian  theory  of  inde- 
pendent unit-characters  accounts  for  this  re- 
sult fully.  No  other  hypothesis  has  as  yet  been 
suggested  which  can  account  for  it. 

Suppose  that  each  unit  has  a  different  mate- 
rial basis  in  the  gamete.  Let  us  represent  the 
material  basis  of  hair-length  by  a  circle,  that 
of  hair-color  by  a  square,  then  combinations 
and  recombinations  arise  as  shown  in  Fig.  30. 
The  composition  of  the  gametes  furnished  by 
the  parents  is  shown  in  the  first  line  of  the 
figure;  that  of  an  F±  individual  (or  zygote), 
in  the  second  line;  that  of  the  gametes 
formed  by  the  F±  individual  in  the  third  line. 
L  meets  S  and  W  meets  D  in  fertilization  to 
form  an  Fx  individual  double  and  also  hetero- 
zygous as  regards  hair  length  and  hair  color, 
but  these  units  segregate  again  as  the  gametes 
of  the  Fj  individuals  are  formed,  and  it  is  a  mat- 
ter of  chance  whether  or  not  they  are  associated 

44 


MENDEL'S    LAW    OF   HEREDITY 

as  originally,  L  with  W  and  S  with  D,  or  in  a 
new  relationship,  L  with  D  and  S  with  W. 
Hence  we  expect  the  F±  individuals  to  form 
four  kinds  of  gametes  all  equally  numerous,  - 
L  W,  S  D,  L  D,  and  S  W.  By  chance  unions  of 
these  in  pairs  nine  kinds  of  combinations  be- 
come possible,  and  their  chance  frequencies  will 
be  as  shown  in  Fig.  31.  Four  of  these  com- 
binations, including  nine  individuals,  will  show 
the  two  dominant  characters,  short  and  dark; 
two  classes,  including  three  individuals,  will 
show  one  dominant  and  one  recessive  charac- 
ter, viz.  dark  and  long;  two  more  classes,  in- 
cluding three  individuals,  will  show  the  other 
dominant  and  the  other  recessive  character, 
viz.  short  and  white;  and  lastly,  one  class,  in- 
cluding a  single  individual,  will  show  the  two 
recessive  characters,  long  and  white.  The  four 
apparent  classes,  or,  as  Johannsen  calls  them, 
phenotypes,  will  accordingly  be  as  9:3:3:1. 
This  is  called  the  normal  Mendelian  ratio  for 
a  dihybrid  cross,  —  that  is,  a  cross  involving 
two  unit-character  differences. 

One  individual  in  each  of  these  four  classes 
will,  if  mated  with  an  individual  like  itself, 

45 


HEEEDITY 

breed  true,  for  it  is  homozygous,  containing 
only  like  units.  The  double  recessive  class, 
long  white,  of  course  contains  only  homozygous 
individuals,  but  in  each  class  which  shows  a 
dominant  unit,  heterozygous  individuals  out- 
number homozygous  ones,  as  2  : 1  or  8  : 1.  Now 
the  breeder  who  by  means  of  crosses  has  pro- 
short  Dark.  Long  Dark.  Short  White.  Long  White. 

1  S  D.  S  D         i  L  D.  L  D         i  S  w-  S  w        i  L  w.  L  w 
s  s  D.  L  D        s  L  D.  L  w        s  s  w-  L  w 

2  S  D.  S  w 
_£S  D.  L  w 

933  1 

FIG.  31.  —  Diagram  showing  the  kinds  and  relative  frequencies 
of  the  young  to  be  expected  in  F2  from  the  crossing  of 
animals  shown  in  Figs.  26  and  27. 

duced  a  new  type  of  animal  wishes,  of  course, 
to  "  fix  "  it,  —  that  is,  to  obtain  it  in  a  condi- 
tion which  will  breed  true.  He  must,  therefore, 
obtain  homozygous  individuals.  If  he  is  deal- 
ing with  a  combination  which  contains  only 
recessive  characters,  this  will  be  easy  enough, 
for  such  combinations  are  invariably  homozy- 
gous. His  task  will  become  increasingly  diffi- 
cult the  more  dominant  characters  there  are 
included  in  the  combination  which  he  desires 
to  fix. 

46 


FIG.  32.  —  A  long-haired,  rough  albino  guinea-pig ; 
male,  2002. 


MENDEL'S    LAW   OF   HEREDITY 

The  most  direct  method  for  him  to  follow  is 
to  test  by  suitable  matings  the  unit-character 
constitution  of  each  individual  which  shows  the 
desired  combination  of  characters,  and  to  reject 
all  which  are  not  homozygous.  In  this  way  a 
pure  race  may  be  built  up  from  individuals 
proved  to  be  pure.  Such  a  method,  however, 
though  sure,  is  slow  in  cases  where  the  desired 
combination  includes  two  or  more  dominant 
unit-characters,  for  it  involves  the  application 
of  a  breeding  test  to  many  dominant  individ- 
uals, most  of  which  must  then  be  rejected.  It 
is  therefore  often  better  in  practice  to  breed 
from  all  individuals  which  show  the  desired 
combination,  and  eliminate  from  their  offspring 
merely  such  individuals  as  do  not  show  that 
combination.  The  race  will  thus  be  only  grad- 
ually purified,  but  a  large  stock  of  it  can  be 
built  up  much  more  quickly. 

We  may  next  discuss  a  cross  in  which  three 
unit-character  differences  exist  between  the  par- 
ents, instead  of  two.  If  guinea-pigs  are  crossed 
which  differ  simultaneously  in  three  unit-char- 
acters, color,  length,  and  direction  of  the  hair, 
a  still  larger  number  of  phenotypes  is  obtained 

47 


HEREDITY 

in  F2,  namely,  eight.  A  cross  between  a  short- 
haired,  dark,  smooth  guinea-pig  (compare  Fig. 
22)  and  one  which  was  long-haired,  white,  and 
rough  (Fig.  32)  produced  offspring  in  F±  which 
were  short-haired,  dark,  and  rough  (compare 
Fig.  24),  these  being  the  three  dominant  char- 
acters, two  derived  from  one  parent,  one  from 
the  other.  The  F2  offspring  were  of  eight  dis- 
tinct types,  two  like  the  respective  grandpar- 
ents, one  like  the  F±  individuals  (parents),  and 
the  other  five  new,  shown  in  Fig.  33.  They  are 
short  white  rough,  short  white  smooth,  long 
white  smooth,  long  dark  rough,  and  long  dark 
smooth.  The  largest  of  the  eight  apparent 
classes  (phenotypes)  was  the  one  which  mani- 
fested the  three  dominant  characters,  short, 
dark,  and  rough,  which  had  been  the  exclusive 
F±  type;  the  smallest  class  was  the  one  which 
manifested  the  three  recessive  characters,  long, 
white,  and  smooth.  Theoretically  these  two 
classes  should  be  to  each  other  as  27  : 1.  Of 
the  twenty-seven  triple-dominants,  twenty-six 
should  be  heterozygous. 

A  comparison  of  this  case  with  the  one  just 
previously  described  shows  what  an  increas- 

48 


FIG.  33.  —  Five  new  combinations  of  unit-characters  obtained  in  gen- 
eration F2,  by  crossing  the  animal  shown  in  Fig.  32  with  animals 
like  that  shown  in  Fig.  22. 


MENDEL'S   LAW   OF   HEREDITY 

ingly  difficult  thing  it  is  to  fix  types  obtained  by 
crossing,  if  the  number  of  dominant  characters 


OamctOR 


Zygut* 


Gametes 


FIG.  34.  —  Diagram  to  show  the  gametic  combinations  and 
segregations  involved  in  a  cross  between  guinea-pigs  differ- 
ing in  three  unit-characters.  L  stands  for  long  hair,  S  for 
short  hair,  W  for  white  hair,  and  D  for  dark  hair;  R  for 
rough,  and  Sm  for  smooth  coat.  Compare  Figs.  22  and  32. 

in  the  selected  type  increases.  On  the  theory  of 
unit-characters  the  gametic  combinations  and 
segregations  involved  in  this  cross  are  as  shown 

49 


HEREDITY 

in  Fig.  34.  The  nature  of  the  gametes  formed 
by  the  parents  crossed  is  shown  in  the  first  row ; 
the  composition  of  the  Fx  individuals,  immedi- 
ately below.  In  the  two  lower  rows  are  shown 
four  different  sorts  of  gametic  splittings  which 
may  occur  in  the  Fx  individuals,  producing  thus 
eight  different  kinds  of  gametes.  If,  in  re- 
ality, the  F!  individuals  form  eight  kinds  of 
gametes,  all  equally  numerous,  and  chance 
unions  in  pairs  occur  among  them,  there  should 
be  produced  eight  corresponding  sorts  of  indi- 
viduals numerically  as  27:9:9:9:3:3:3:1. 
In  a  total  of  64  individuals  there  should  be  on 
the  average  one  pure  individual  in  each  of  the 
eight  different  classes.  The  class  numerically  27 
in  64  manifests  three  dominant  characters ;  those 
which  are  numerically  9  in  64  manifest  two  domi- 
nant characters ;  those  which  are  numerically  3 
in  64  manifest  one  dominant  character.  Among 
each  of  these  there  will  be  on  the  average  one 
pure  individual,  but  the  class  which  contains  1 
individual  in  64  is  a  pure  recessive,  for  it  contains 
no  dominant  character.  This  combination,  then, 
requires  no  fixation.  It  will  breed  true  from  the 
start. 

50 


MENDEL'S    LAW    OF    HEREDITY 


BIBLIOGRAPHY 

CASTLE,  W.  E. 

1903.  "  Mendel's  Law  of  Heredity."  Proc.  Am.  Acad. 
Arts  and  Sci.,  38,  pp.  535-548;  also,  Science,  N.  S.,  18, 
pp.  396-406. 

1905.     "  Heredity  of  Coat-characters  in  Guinea-pigs  and 
Rabbits."     Carnegie  Institution  of  Washington,  Publica- 
tion No.  23,  78  pp.,  6  pi. 
FARABEE,  W.  C. 

1905.  "Inheritance  of  Digital  Malformations  in  Man." 
Papers  of  the  Peabody  Museum,  Harvard  University,  vol. 
3,  No.  3,  pp.  69-77,  5  pi. 


CHAPTER   IV 

THE  DETERMINATION  OF  DOMINANCE;  HETEROZY- 
GOUS  CHARACTERS  AND  THEIR  FIXATION;  ATA- 
VISM OR  REVISION 

WE  have  noticed  that  when  a  black 
guinea-pig  of  pure  race  is  mated 
with  a  white  one,  only  black  off- 
spring are  produced;  and  that  when  rough- 
coated  guinea-pigs  are  mated  with  smooth- 
coated  ones,  only  rough-coated  young  are  pro- 
duced; and  that  when  short-haired  guinea- 
pigs  are  mated  with  long-haired  ones,  only 
short-haired  young  are  produced.  The  char- 
acter which  in  each  case  is  seen  in  the  young 
we  call  dominant,  that  which  is  unseen  we 
call  recessive.  Thus  black  is  dominant  over 
white,  rough  coat  over  smooth  coat,  and  short 
coat  over  long  coat. 

A  question  which  has  given  much  concern  to 
students  of  heredity  is  this,  —  upon  what  does 

52 


DOMINANCE 

dominance  depend?  Why  should  black  domi- 
nate white  rather  than  the  reverse  f 

In  poultry,  indeed,  the  relations  are  often 
reversed,  white  dominating  black.  Why  is 
this?  Several  attempted  explanations  have 
been  made,  but  none  of  them  is  thoroughly 
satisfactory.  The  one  which  has  found  wid- 
est acceptance  is  this:  In  the  dominant  in- 
dividual something  is  present  which  is  want- 
ing in  the  recessive.  Thus,  in  the  black  guinea- 
pig  there  is  present  some  ferment-like  body 
or  some  ingredient  of  black  which  is  wanting 
in  the  albino.  Accordingly,  the  black  guinea- 
pig  forms  pigment,  a  thing  which  the  albino 
can  do  only  feebly  or  not  at  all.  The  distinc- 
tive something  of  the  black  parent  therefore 
dominates  a  corresponding  nothing  of  the 
white  parent.  White  fowls,  on  the  other  hand, 
are  not  albinos.  They  have  pigmented  eyes. 
Accordingly  they  do  not  lack  the  power  to 
form  pigment,  owing  to  the  absence  of  -some 
necessary  ferment  or  pigment  ingredient. 

White  guinea-pigs  occur  which  are  in  a  way 
comparable  with  white  fowls.  They  look  ex- 
actly like  albinos,  except  that  their  eyes  are 
5  53 


HEREDITY 

black,  whereas  the  eyes  of  the  albino  are  pink. 
If  such  a  black-eyed  white  guinea-pig  is  crossed 
with  an  albino  of  the  sort  shown  in  Fig.  15, 
the  young  produced  will  be  black  all  over. 
Now  this  result  shows  that  the  black-eyed 
white  animal  possesses  what  is  lacking  in  the 
albino  as  compared  with  the  all-black  animal. 
It  would  seem,  therefore,  that  it  lacks  some- 
thing different  from  what  the  albino  lacks,  and 
that  a  cross  of  the  two  supplies  both  lacks,  the 
albino  supplying  what  is  wanting  in  the  black- 
eyed  white,  and  vice  versa.  Accordingly,  wholly 
black  offspring  result  from  the  crossing  of  the 
two  white  races. 

But  the  case  of  white  poultry  is  different 
from  this,  since  white  poultry  lack  nothing 
that  is  necessary  to  produce  the  complete 
black  plumage.  For  when  white  fowls  crossed 
with  black  ones  produce  white  offspring,  if 
these  offspring  are  then  bred  with  each  other, 
they  produce  both  white  offspring  and  black 
ones  in  the  ratio  3  to  1.  White  fowls,  there- 
fore, are  able  to  produce  the  black  condition. 
This  ability  is  in  the  white  individual  held  in 
abeyance,  it  is  not  exercised.  Why,  we  do 

54 


DOMINANCE 

not  know.  Some  suppose  it  to  be  held  in 
check  by  an  additional  unit-character,  an  in- 
hibiting factor,  but  we  have  no  direct  evidence 
that  such  a  factor  exists.  All  that  we  are 
warranted  in  saying  at  the  present  time  is 
that  black  and  white  in  poultry  represent  dif- 
ferent conditions  of  pigmentation,  alternative 
to  each  other  in  heredity.  In  crosses  of  the 
two,  white  is  ordinarily  dominant  over  black, 
but  in  crosses  between  certain  strains  of  white 
and  black  poultry  this  relationship  is  reversed, 
as  Bateson  has  shown. 

In  still  other  cases,  a  cross  of  white  with 
black  fowls  produces  offspring  which  resemble 
neither  parent  closely,  but  which  are  in  reality 
intermediate.  They  are  known  as  blue  or 
Andalusian  fowls.  They  manifest  a  dilute 
condition  of  black,  such  as  one  might  obtain 
by  mixing  lampblack  with  flour;  they  are  in 
reality  a  fine  mosaic  of  black  with  white. 
Such  a  condition  has  thus  far  been  obtained 
only  from  a  cross  of  black  fowls  with  a  pecu- 
liar strain  of  impure  sooty  whites.  This  strain 
undoubtedly  contains  the  mosaic  pattern  but 
without  sufficient  black  pigment  to  make  it 

55 


HEREDITY 

plainly  visible.  A  cross  with  a  black  race 
makes  it  visible.  No  one,  however,  has  suc- 
ceeded in  "  fixing  ''  a  blue  race,  that  is, 
in  obtaining  a  strain  which  would  breed 
true. 

When  two  blue  individuals  are  bred  together 
they  produce  black,  blue,  and  white  offspring 
in  the  ratio  1:2:1.  The  blacks  are  homo- 
zygous,  B  B ;  the  whites  also  are  homozygous, 
WW,  but  the  blues  are  invariably  heterozy- 
gous, B  W.  Blue  accordingly  in  this  case  is 
called  a  heterozygous  character,  one  which  is 
due  to  the  presence  in  one  zygote  of  two  unlike 
unit-characters,  which  invariably  segregate  from 
each  other  at  the  genesis  of  gametes,  but  which 
jointly  produce  a  different  appearance  from 
what  either  produces  by  itself.  If  a  strain  of 
Andalusian  fowls  should  ever  be  secured  which 
would  breed  true,  it  would  have  to  come  about 
by  the  association  of  black  with  white  in  a 
non-segregating  relationship,  so  that  both 
would  be  transmitted  in  the  same  gamete. 
That  is,  one  would  have  to  secure  in  the  same 
gamete  with  white  enough  black  pigment  to 
bring  out  the  latent  mosaic  pattern,  and  fur- 

56 


DOMINANCE 

ther,  one  would  have  to  secure  a  homozygous 
race  of  fowls  which  formed  such  gametes. 

Success  would  be  most  likely  to  attend  the 
experiment  if  one  selected  always  the  sootiest 
whites  obtained  from  blue  parents,  for  blue 
results,  as  we  have  seen,  from  the  association 
of  more  black  with  the  white  and  in  the  pat- 
tern borne  already  by  the  white  race. 

A  much-debated  case  of  inheritance  which 
involves  this  principle  of  unfixable  heterozy- 
gous characters  occurs  among  fancy  mice,  in 
the  variety  known  as  yellow.  A  wonderful 
series  of  color  varieties  exists  among  mice 
kept  as  pets,  equalling  or  perhaps  surpassing 
that  known  in  the  case  of  any  other  mammal. 
All  these  varieties  appear  to  be  derivatives 
of  the  common  house-mouse,  with  which  they 
cross  readily.  All  are  capable  of  explanation 
as  unit-character  variations  from  the  condition 
of  the  house-mouse.  Among  all  these  varie- 
ties yellow  is  most  peculiar  in  its  behavior. 
In  crosses  it  is  dominant  over  all  others,  yet 
is  itself  absolutely  unfixable. 

If  certain  strains  of  yellow  mice  are  crossed 
with  black  ones,  the  offspring  produced  are  of 

57 


HEEEDITY 

two  sorts  equally  numerous,  yellow  and  black. 
From  this  result  alone  it  is  impossible  to  say 
which  is  the  dominant  character,  but  breeding 
tests  of  the  offspring  show  that  yellow  is  the 
dominant  character.  For  the  black  offspring 
bred  together  produce  only  black  offspring, 
but  the  yellows  bred  together  produce  both 
yellow  offspring  and  black  ones.  The  curious 
feature  of  the  case  is  that  when  yellows  are 
bred  with  each  other  no  pure  yellows,  that  is, 
homozygous  ones,  are  obtained.  Hundreds  of 
yellow  individuals  have  been  tested,  but  the 
invariable  result  has  been  that  they  are  found 
to  be  heterozygous ;  that  is,  they  transmit  yel- 
low in  half  their  gametes,  but  some  other  color 
in  the  remaining  gametes  —  it  may  be  black, 
or  it  may  be  brown,  or  it  may  be  gray.  The 
black,  brown,  or  gray  animals  obtained  by 
mating  yellow  with  yellow  mice  never  produce 
yellow  offspring  if  mated  with  each  other. 
This  shows  that  they  are  genuine  recessives 
and  do  not  contain  the  yellow  character,  which 
is  dominant. 

Now  ordinary  heterozygous  dominants,  when 
mated  with  each  other,  produce  three   domi- 

58 


DOMINANCE 

nant  individuals  to  one  recessive.  Accordingly 
we  should  expect  yellow  mice,  if,  as  stated, 
they  are  invariably  heterozygous,  to  produce 
three  yellow  offspring  to  one  of  a  different 
color,  but  curiously  enough  they  do  not.  They 
produce  two  yellows  (instead  of  the  expected 
three)  to  every  one  of  a  different  color.  About 
the  ratio  there  can  be  no  reasonable  doubt. 
It  has  been  determined  with  great  accuracy 
by  my  pupil,  Mr.  C.  C.  Little,  who  finds  that 
in  a  total  of  over  twelve  hundred  young  pro- 
duced by  yellow  parents  almost  exactly  two- 
thirds  are  yellow.  Instead  of  the  regular 
Mendelian  ratio  3  : 1,  we  have  then  in  this  case 
the  peculiar  ratio  2  : 1,  and  this  requires  ex- 
planation. The  explanation  of  this  ratio  is  to 
be  found  in  the  same  circumstance  as  is  the 
total  absence  of  pure  yellows.  Pure  yellow 
zygotes  are  indeed  formed,  but  they  perish  for 
some  unaccountable  reason.  For  a  yellow  in- 
dividual forms  gametes  of  two  sorts  with  equal 
frequency,  viz.  yellow  and  non-yellow  (let  us 
say  black).  For,  if  yellow  individuals  are 
mated  with  black  ones,  half  the  offspring  are 
black,  half  yellow,  as  already  stated. 

59 


HEREDITY 

If  now  yellow  individuals  are  mated  with 
each  other  we  expect  three  sorts  of  young  to 
be  produced  numerically,  as  1:2:1,  viz.  1  Y  Y, 
2  Y B,  and  IBB.  But  since  observation  shows 
that  only  two  combinations  are  formed  which 
contain  yellow  to  one  not  containing  yellow, 
and  since  further  all  yellows  which  survive 
are  found  to  be  heterozygous  (YB),  it  must 
be  that  the  expected  YY  individual  either  is 
not  produced  or  straightway  perishes.  As  to 
which  of  these  two  contingencies  happens  we 
also  have  experimental  evidence.  Mr.  Little 
finds  that  yellow  mice  when  mated  to  black 
ones  produce  larger  litters  of  young  than  when 
they  are  mated  to  yellow  ones.  The  average- 
sized  litter  contains  something  like  5.5  young 
when  the  mate  is  a  black  animal,  but  only  4.7 
when  it  is  a  yellow  animal.  It  is  evident,  then, 
that  about  one  young  one  out  of  a  litter  per- 
ishes when  both  parents  are  yellow,  and  this 
undoubtedly  is  the  missing  yellow-yellow  zy- 
gote.  The  yellows  which  are  left  are  hetero- 
zygous yellow-black  zygotes,  and  they  are  to 
those  that  perish  as  2:1.  They  are  also  to 
the  non-yellow  zygotes  as  2:1,  the  ratio  ob- 

60 


DOMINANCE 

served    also    among   the    surviving   young   of 
yellow  by  yellow  parents. 

This  interpretation  of  the  2  : 1  ratio  observed 
in  this  case  is  strongly  supported  by  a  similar 
case  among  plants,  in  which  the  evidence  is 
even  more  complete.  A  so-called  "  golden  ' 
variety  of  snapdragon,  one  in  which  the  foli- 
age was  yellow  variegated  with  green,  was 
found  by  the  German  botanist,  Baur,  to  be 
unfixable,  producing  when  self-pollinated  fully 
green  plants  as  well  as  golden  ones,  in  the 
ratio  2  golden  :  1  green.  The  green  plants  were 
found  to  breed  true,  that  is,  to  be  recessives, 
while  the  golden  ones  were  invariably  found 
to  be  heterozygous.  Baur  found,  however,  by 
germinating  seeds  of  golden  plants  very  care- 
fully, that  there  were  produced  in  addition  to 
green  plants  and  golden  ones  a  few  feeble 
seedlings  entirely  yellow,  not  variegated  with 
green,  as  the  golden  plants  are.  These,  for 
lack  of  assimilating  organs  (green  chlorophyl), 
straightway  perished.  Clearly  they  were  the 
missing  pure  yellow  zygotes. 

Some  Mendelian  characters,  while  not  them- 
selves heterozygous  and  so  unfixable,  are  never- 

61 


HEREDITY 

theless  produced  only  when  two  independently 
inherited  factors  are  present  together.  A 
character  of  this  sort  does  not  itself  conform 
with  the  simple  Mendelian  laws  of  inheritance, 
but  its  factors  do.  Herein  lies  the  explanation 
of  atavism  or  reversion,  and  the  process  by 
which  reversionary  characters  may  be  fixed. 

Atavism  or  reversion  to  an  ancestral  con- 
dition is  a  phenomenon  to  which  Darwin  re- 
peatedly called  attention.  He  realized  that  it 
is  a  phenomenon  which  general  theories  of 
heredity  must  account  for.  He  supposed  that 
the  environment  was  chiefly  responsible  for  the 
reappearance  in  a  species  of  a  lost  ancestral 
condition,  but  that  in  certain  cases  the  mere 
act  of  crossing  may  reawaken  slumbering  an- 
cestral traits.  Thus  he  noticed  that  when 
rabbits  of  various  sorts  are  turned  loose  in  a 
warren  together,  they  tend  to  revert  to  the 
gray-coated  condition  of  wild  rabbits.  And 
when  pigeons  are  crossed  in  captivity  they 
frequently  revert  to  the  plumage  condition  of 
the  wild  rock  pigeon,  Columba  livia.  In  plants, 
too,  Darwin  recognized  that  crossing  is  a  fre- 
quent cause  of  reversion.  The  explanation 

62 


DOMINANCE 

which  he  gave  was  the  best  that  the  knowl- 
edge of  his  time  afforded,  but  it  leaves  much 
to  be  desired.  This  lack,  however,  has  been 
completely  supplied  by  the  Mendelian  princi- 
ples. An  illustration  or  two  may  now  be 
cited. 

When  pure-bred  black  guinea-pigs  are  mated 
with  red  ones,  only  black  offspring  are  as  a 
rule  obtained.  The  hairs  of  the  offspring  do 
indeed  contain  some  red  pigment,  but  the  black 
pigment  is  so  much  darker  that  it  largely 
obscures  the  red.  In  other  words,  black  be- 
haves as  an  ordinary  Mendelian  dominant.  In 
the  next  generation  black  and  red  segregate 
in  ordinary  Mendelian  fashion,  and  the  young 
produced  are  in  the  usual  proportions,  three 
black  to  one  red,  or  1 : 1  in  back-crosses  of  the 
heterozygous  black  with  red.  All  black  races 
behave  alike  in  crosses  with  the  same  red  in- 
dividual, but  among  red  animals  individual 
differences  exist.  Some,  instead  of  behaving 
like  Mendelian  recessives,  produce  in  crosses 
with  a  black  race  a  third  apparently  new  con- 
dition, but  in  reality  a  very  old  one,  the  agouti 
type  of  coat  found  in  all  wild  guinea-pigs,  as 

G3 


HEREDITY 

well  as  in  wild  rats,  mice,  squirrels,  and  other 
rodents.  In  this  type  of  coat  reddish  yellow 
pigment  alone  is  found  in  a  conspicuous  band 
near  the  tip  of  each  hair,  while  the  rest  of  the 
hair  bears  black  pigment.  The  result  is  a 
brownish  or  grayish  ticked  or  grizzled  coat, 
inconspicuous,  and  hence  protective  in  many 
natural  situations. 

Some  red  individuals  produce  the  reversion 
in  half  of  their  young  by  black  mates,  some 
in  all,  and  others,  as  we  have  seen,  in  none, 
this  last  condition  being  the  commonest  of  the 
three.  It  is  evident  that  the  reversion  is  due 
to  the  introduction  of  a  third  factor,  additional 
to  simple  red  and  simple  black.  It  is  evident 
further  that  this  new  third  factor,  which  we 
will  call  A  (agouti),  has  been  introduced 
through  the  red  parent,  and  that  as  regards 
this  factor,  A,  some  individuals  are  homozy- 
gous  (AA)  in  character,  others  are  heterozy- 
gous (transmit  it  in  half  their  gametes  only), 
while  others  lack  it  altogether.  Further  ob- 
servations show  that  it  is  independent  in  its 
inheritance  of  both  black  and  red;  it  is  in 
fact  an  independent  Mendelian  character,  which 

64 


DOMINANCE 

can  become  visible  only  in  the  presence  of  both 
black  and  red,  because  it  is  a  mosaic  of  those 
two  pigments.  If  the  Fx  agouti  individuals 
are  bred  together  they  produce  in  the  next 
generation  (F2)  three  sorts  of  young,  viz. 
agouti,  black,  and  red,  which  are  numerically 
as  9:3:4.  This  evidently  is  a  modification  of 
the  dihybrid  Mendelian  ratio  9:3:3:1,  result- 
ing from  the  fact  that  the  last  two  classes  are 
superficially  alike.  They  are  red  animals  with 
and  without  the  agouti  factor  respectively;  but 
this  agouti  factor  is  invisible  in  the  absence 
of  black,  so  that  both  sorts  of  reds  look  alike. 
Together  they  number  four  in  sixteen  of  the 
Fo  offspring. 

Fig.  35  is  intended  to  show  how  the  inde- 
pendent factors  behave  in  heredity.  The  black 
parent  contributes  the  factor  B,  the  red  par- 
ent, R  and  A,  so  that  the  zygote,  or  n£w  indi- 
vidual, contains  the  three  factors  necessary  to 
the  production  of  agouti.  When  the  new  in- 
dividual forms  gametes  (sex-cells),  these  will 
l)c  of  four  different  kinds,  for  A  is  independ- 
ent of  B  and  of  R  and  may  pass  out  with 
either  one  in  the  reduction  division  which  sepa- 

65 


HEREDITY 


Agouti        Agouti        Agouti        Agouti  (fixed) 
Black          Fed  Black 

Red 
4221 

FIG.  35.  —  Diagram  to  show  the  gametic  combination  and  re- 
combinations which  occur  in  the  production  and  fixation  of 
an  atavistic  coat-character  in  guinea-pigs. 

Row  1  shows  the  character  of  the  gametes  formed  by  the 
parents  crossed;  row  2  shows  the  character  of  the  FI  agouti 
individuals  resulting  from  the  cross;  row  3  shows  the  two 
different  sorts  of  gametic  splittings  which  may  occur  in  the 
production  of  gametes  by  the  FI  agoutis,  and  how  four  differ- 
ent kinds  of  gametes  result;  row  4  shows  how  among  such 
gametes  four  different  kinds  of  unions  may  occur  that  will 
produce  agouti  young.  The  BA-BA  combination,  it  will 
be  understood,  could  result  only  from  the  union  of  a  BA 
gamete  with  another  gamete  of  like  constitution.  Below 
each  of  the  four  combinations  is  indicated  the  kinds  of  young 
which  an  animal  of  that  sort  would  produce  if  mated  with 
an  animal  like  itself.  The  numerals  show  the  expected 
relative  frequencies  of  the  four  sorts  of  combinations. 


66 


DOMINANCE 

rates  B  from  R.  That  division  accordingly 
may  occur  either  so  as  to  form  gametes  B  and 
R  A  respectively,  or  what  is  equally  probable, 
so  as  to  produce  gametes  B  A  and  R.  Obser- 
vation confirms  this  interpretation,  for  it  is 
found  that  the  reversionary  agoutis  do  not 
breed  true,  but  produce  young  of  the  three 
sorts,  agouti,  black,  and  red,  as  expected.  We 
expect  black  individuals  from  unions  of  B  with 
B,  or  of  B  with  R;  we  expect  red  individuals 
from  unions  of  R  with  R  or  with  R  A,  and 
from  unions  of  R  A  with  R  A ;  we  expect  agoutis 
to  be  produced  by  any  gametic  union  which 
brings  together  the  three  factors  B,  R,  and  A. 
There  are  six  chances  in  sixteen  for  the  oc- 
currence of  such  a  union,  when  the  rever- 
sionary agoutis  are  bred  together.  In  fact, 
however,  agoutis  are  produced  much  oftener. 
Approximately  nine  out  of  sixteen  of  the 
young  have  been  found  to  be  agoutis.  The 
unexpected  excess  of  agoutis  in  our  experi- 
ments was  fully  explained  when  these  second- 
generation  agoutis  were  tested  individually. 
It  was  then  found  that  they  are  of  four  sorts 
as  regards  breeding  capacity.  The  first  sort 

67 


HEEEDITY 

produces  the  three  kinds  of  young,  agouti,  black, 
and  red;  the  second  sort  produces  only  agouti 
young  and  red  young;  the  third  sort  produces 
only  agouti  young  and  black  young.  The  fourth 
sort  produces  only  agouti  young,  i.  e.  repre- 
sents the  fully  fixed  agouti  type,  the  completely 
recovered  wild  type. 

In  the  chart  (Fig.  35)  are  indicated  cer- 
tain gametic  unions  which  would  lead  to  the 
production  of  these  four  classes  of  agoutis. 
The  probable  frequencies  of  their  occurrence 
on  the  basis  of  chance  are  4:2:2:1. 

Experiment  made  it  clear  that  R  as  an  inde- 
pendent gametic  factor  is  not  necessary  to  the 
production  of  the  agouti  character,  as  was  at 
first  thought  to  be  the  case,  but  that  any 
gametic  union  which  includes  both  B  and  A 
will  produce  an  agouti  individual  whether 
E  is  or  is  not  present.  Yet  a  microscopic 
examination  of  the  agouti  hair  shows  that 
red  pigment  is  present  in  a  distinct  band 
near  the  hair-tip.  As  a  matter  of  fact  all 
black  individuals,  even  when  they  breed  true, 
probably  form  some  red  pigment  along  with 
the  black,  but  its  presence  is  overlooked  when 

68 


DOMINANCE 

the  more  opaque  black  is  distributed  through- 
out the  whole  length  of  the  hair.  When,  how- 
ever, black  is  excluded  from  the  hair-tip,  the 
red  then  becomes  visible  as  the  agouti  mark- 
ing; elsewhere  the  hair  appears  black.  Eed, 
then,  we  may  assume,  is  always  present  with 
black  in  sufficient  quantities  to  produce  the 
agouti  marking  if  the  factor  A  is  present  (ab- 
sence of  black  from  the  hair-tip).  This  ex- 
plains why  blacks  never  give  the  reversion  in 
any  sort  of  cross,  but  it  is  always  brought 
about  through  the  agency  of  the  red  parent. 
If  a  black  individual  contained  the  factor  A, 
it  would  no  longer  be  a  black  individual,  but 
an  agouti  one. 

The  existence  of  a  third  factor,  A,  in  cases 
of  reversion  in  coat-character  among  rodents 
was  long  overlooked  merely  because  it  does 
not  represent  a  distinct  pigment  or  set  of  pig- 
ments, but  consists  in  a  particular  kind  of 
pigment  distribution  on  the  individual  hairs. 
The  agouti  hair  is  due  to  a  definite  cycle  of 
activity  of  the  hair  follicle  in  forming  its  pig- 
ments,—  first  black,  then  red,  then  black;  the 
wholly  black  hair  is  due  to  a  continuous  process 
6  69 


HEEEDITY 

of  pigment  formation  without  alternation  in  the 
character  of  the  pigments  produced. 

In  rabbits  as  well  as  in  guinea-pigs  rever- 
sion to  the  original  wild  type,  in  this  case 
gray,  may  be  obtained  by  crossing  a  black  ani- 
mal with  a  yellow  one.  In  guinea-pigs  the 
yellow  (or  red)  animal  which  will  yield  this 
result  cannot  be  distinguished  in  appearance 
from  one  which  will  not;  but  in  rabbits  the 
yellow  animal  which  will  give  reversion  has 
a  white  belly  and  tail,  while  the  one  which 
will  not  give  reversion  is  not  so  distinguished. 

We  now  know  what  is  implied  in  the  fixa- 
tion of  a  heterozygous  character  obtained  by 
crossing.  When  A  and  B  are  crossed  we  ob- 
tain a  third  condition,  C.  C  is  due  either  to 
the  simple  coexistence  of  A  with  B,  or  to  the 
coexistence  with  them  of  a  third  factor  intro- 
duced with  one  or  the  other.  In  either  case 
fixation  will  consist  in  getting  into  the  gamete 
all  the  factors  necessary  to  the  production  of 
C.  In  the  first  supposed  case  the  zygote  is 
A-B  and  the  resultant  is  equivalent  to  C.  Fix- 
ation will  consist  in  getting  a  zygote  of  the 
formula  AB  •  AB.  In  the  second  supposed 

70 


DOMINANCE 

case  the  zygote  produced  is  either  A-CB  or 
AC-B;  fixation  will  consist  in  obtaining  a  zy- 
gote ACB-ACB;  every  gamete  formed  will 
then  contain  the  three  factors  A,  C,  and  B. 

BIBLIOGEAPHY 

BATESON,  W. 

1909.  "Mendel's  Principles  of  Heredity,"  393  pp.,  illus- 
trated.   University  Press,  Cambridge;  also  G.  P.  Put- 
nam's Sons,  N.  Y.    [Contains  translation  of  Mendel's 
original  papers.] 

BAUR,  E. 

1907.    "  Untersuchungen  iiber  die  Erblichkeitsverhaltnisse 
einer  nur  in  Bastardform  lebensfahigen  Sippe  von  An- 
tirrhinum majus."    Ber.  d.  Deuisch  Bot.  Gesellsch.,  25, 
p.  442. 
CASTLE,  W.  E. 

1907.    "On  a  Case  of  Reversion  Induced  by  Cross-Breed- 
ing and  its  Fixation."      Science,  N.  S.,  25,  pp.  151-153. 

1907.  "The  Production  and  Fixation  of  New  Breeds." 
Proc.  Amer.  Breeders'  Ass'n,  3,  pp.  34-41. 

CASTLE,  W.  E.  and  LITTLE,  C.  C. 

1910.  "On  a  Modified  Mendelian  Ratio  Among  Yellow 
Mice.  "    Science,  N.  S.}  32,  pp.  868-870. 

CUENOT,  L. 

1908.  "Sur    quelques   anomalies   apparentes   des   pro- 
portions Mendelie"nnes."    Arch.  Zool.  Exper.  (4),  Notes 
et  Revue,  p.  vii. 

DAVENPORT,  C.  B. 

1906.      "Inheritance    in  Poultry."      Carnegie  Institution 
of  Washington,  Publication  No.  52,  104  pp.,  17  pi. 

1909.  "Inheritance  of  Characteristics  in  Domestic  Fowl." 
Carnegie    Institution    of   Washington,    Publication    No. 
121,  100  pp.,  12  pi. 

71 


CHAPTER   V 

EVOLUTION    OF    NEW    EACES    BY    LOSS    OB    GAIN    OF 
CHARACTERS 

OUR  knowledge  of  Mendelian  phenom- 
ena is  most  complete  in  the  case  of 
color-inheritance.  We  find  that  the 
flower-colors  of  plants  and  the  coat-colors  of 
mammals  are  alike  complex,  and  that  what 
seem  at  first  sight  simple  results  may  really 
depend  on  several  independent  factors  acting 
conjointly.  By  analysis  of  such  complex  cases 
we  are  able  to  gain  some  idea  of  what  the 
probable  course  of  evolution  has  been  in  the 
production  of  the  color  varieties  found  among 
cultivated  plants  and  domesticated  animals. 

Thus  among  rodents  (mice,  rabbits,  guinea- 
pigs)  the  coat  is  grayish,  consisting  of  black, 
brown,  and  yellow  pigments  mingled  together 
on  the  same  individual  hair  in  a  pattern  of 
greater  or  less  complexity. 

72 


EVOLUTION   BY  LOSS 

The  simplest  variation  from  this  ancestral 
type  of  coloration  is  albinism,  a  wholly  unpig" 
mented  condition  in  which  the  eyes  are  pink. 
This  is  due  to  the  loss  of  the  capacity  to  form 
pigment.  Albinism  is  recessive  in  crosses.  We 
explain  it  by  assuming  that  something  neces- 
sary for  color  production  is  wanting  in  the 
albino,  and  call  that  something  the  color-factor 
C,  without  necessarily  making  any  assumption 
as  to  its  nature.  Another  common  variation 
is  the  loss  of  the  pattern-factor  of  the  indi- 
vidual hair,  the  agouti  or  A  factor.  An  ac- 
count of  the  discovery  of  this  factor  was  given 
in  the  last  chapter.  In  consequence  of  the  loss 
of  this  factor  the  pigments  become  mingled 
together  without  order,  and  the  result  is  a 
uniform  black,  the  denser  pigment  hiding  the 
others. 

A  third  variation  is  the  loss  of  the  capacity 
to  form  black  pigment  (factor  B),  only  brown 
and  yellow  pigments  being  left.  Thus  arise 
brown  and  cinnamon  varieties.  Through  these 
three  independent  loss-variations  there  arise 
eight  different  color-varieties  as  follows: 


73 


HEEEDITY 

Gray  (or  agouti)  =  C  B  Br  A;  Cinnamon  =  C  Br  A 

Black  =  C  B  Br;  Brown        =  C  Br 

Albino  (1)  =  B  Br  A;  Albino  (3)  =  Br  A 

Albino  (2)  =  B  Br;  Albino  (4)  =  Br 

Proof  of  the  correctness  of  this  interpretation 
may  be  obtained  from  crosses.  Suppose  the 
four  kinds  of  albinos  described  be  crossed  with 
the  same  colored  variety,  brown;  albino  1  will 
produce  gray  offspring,  albino  2  will  produce 
black  ones,  albino  3  will  produce  cinnamon 
ones,  and  albino  4  will  produce  brown  ones. 
The  cross  with  albino  1  brings  together  all  the 
four  factors  entering  into  the  production  of 
gray,  viz.  C,  B,  Br,  and  A,  hence  the  young  are 
gray.  The  cross  with  albino  2  brings  together 
the  factors  C,  B,  and  Br  only.  The  result  is 
black.  The  cross  with  albino  3  brings  together 
the  factors  C,  Br,  and  A;  result,  a  cinnamon 
animal.  The  cross  with  albino  4  brings  to- 
gether no  factors  except  C  and  Br;  result,  a 
brown  animal. 

Thus  far  we  have  considered  merely  varia- 
tions which  arise  by  loss  of  one  or  more  of 
the  three  unit-characters,  A,  B,  and  C.  We 
may  now  consider  variations  which  arise 

74 


EVOLUTION   BY   LOSS 

by  modification  without  loss  of  these  same 
factors. 

Yellow  varieties  owe  their  origin  to  a  re- 
duction in  the  amount  of  black  or  brown  pig- 
ment in  the  fur,  and  to  a  corresponding  in- 
crease in  the  amount  of  yellow.  In  some 
yellow  animals,  such  as  the  sooty  yellow  rab- 
bit, black  and  brown  pigments  are  not  wholly 
lacking  in  the  fur,  but  are  only  greatly  re- 
duced in  amount.  They  always  persist  in 
the  eye.  In  other  yellow  animals,  mice  for 
example,  the  black  or  brown  pigments  are 
wholly  absent  from  the  fur,  and  they  may  also 
be  greatly  reduced  in  amount  in  the  eye,  as 
in  the  variety  known  as  pink-eyed  yellow,  but 
in  no  yellow  animal,  so  far  as  I  am  aware,  is 
the  production  of  black  and  of  brown  pigments 
wholly  suppressed. 

In  any  mammal  which  possesses  yellow  varie- 
ties we  can  produce  by  suitable  crosses  as  many 
different  varieties  of  yellows  as  there  are  of 
gray,  black,  cinnamon,  and  brown  varieties 
combined.  For  example,  in  mice,  yellow  indi- 
viduals of  which,  as  was  shown  in  the  last 
chapter,  are  invariably  heterozygous  and  pro- 

75 


HEEEDITY 

duce  some  other  variety  than  yellow,  even  when 
mated  with  yellows,  we  can  recognize  the  fol- 
lowing varieties  distinct  in  breeding  capacity, 
though  all  looking  very  similar. 

1.  Yellows  which  produce  yellow  young  and  gray 
ones; 

2.  Yellows  which  produce  yellow  young'  and  black 
ones; 

3.  Yellows    which     produce    yellow    young    and 
cinnamons; 

4.  Yellows  which  produce  yellow  young  and  brown 
ones. 

Albino  varieties  occur  which  correspond  with 
each  of  these  yellow  varieties,  viz.  (1)  albinos 
which  if  crossed  with  brown  will  produce  yel- 
low young  and  gray  ones;  (2)  albinos  which 
crossed  with  brown  produce  yellow  young  and 
black  ones ;  (3)  albinos  which  crossed  with  brown 
produce  yellow  young  and  cinnamon  ones ;  and 
(4)  albinos  which  crossed  with  brown  produce 
yellow  young  and  brown  ones.  Such  albinos, 
of  course,  differ  from  the  corresponding  yellow 
varieties  merely  by  the  general  color  factor  C, 
which  the  albino  lacks.  If  this  is  added  by  a 
cross,  they  produce  the  same  visible  result  as 

76 


EVOLUTION    BY   LOSS  . 

the  corresponding  yellow  variety  in  the  same 
cross. 

In  addition  to  the  modification  which  pro- 
duces yellow  varieties,  we  can  recognize  sev- 
eral other  modified  conditions  of  the  unit- 
characters  A,  B,  C,  and  Br,  which  modifica- 
tions produce  whole  series  of  color  varieties. 
For  a  modified  condition  of  a  single  unit- 
character  is  capable  of  producing  as  many 
new  varieties  as  there  are  possible  combina- 
tions of  the  modified  character  with  other  unit 
characters. 

One  who  attends  a  poultry-show  cannot  fail 
to  be  impressed  with  the  great  number  of  color 
varieties  among  poultry.  Let  him  first  observe 
these  among  fowls  of  common  size,  and  if  he 
then  visits  the  bantam  section  he  will  find  them 
all  duplicated  in  miniature  among  the  bantams. 
If  a  new  color  variety  is  brought  out,  it  is  only 
a  short  time  until  it  finds  its  place  among  the 
bantams  as  well  as  among  fowls  of  common 
size.  The  dwarf  size  of  the  bantam  is  clearly 
due  to  a  modified  condition  of  one  or  more  unit- 
characters  capable  of  combinations  with  as  many 
different  kinds  of  coloration  as  occur  among 

77 


HEEEDITY 

poultry.  The  various  combinations  are  of 
course  brought  about  by  crossing,  and  two  gen- 
erations suffice  theoretically  for  securing  them. 
In  mice,  if  one  possessed  only  the  albino 
variety  last  described,  —  the  one  which  corre- 
sponds with  the  brown-eyed  yellow  variety,  - 
he  could  easily  produce  within  six  months  every 
one  of  the  various  color  varieties  which  have 
been  mentioned.  All  he  would  have  to  do 
would  be  to  catch  some  wild  mice  and  cross 
these  with  his  albinos.  The  immediate  off- 
spring produced  by  the  cross  might  seem  un- 
promising; they  would  either  be  gray,  exactly 
like  wild  mice,  or  else  yellow.  But  if  our 
breeder  possessed  the  faith  to  breed  a  second 
generation  from  these  animals,  he  would  be 
rewarded  by  seeing  all  the  color  varieties  which 
I  have  described  put  in  an  appearance,  viz. 
yellows  with  black  eyes,  and  yellows  with 
brown  eyes,  blacks,  browns,  cinnamons,  and 
grays,  and  albinos  corresponding  in  character 
with  each  colored  variety  except  for  the  lack 
of  the  color-factor  C. 

It  may  be  of  interest  to  consider  how  some 
additional  color  varieties  of  mice  have  arisen, 

78 


EVOLUTION   BY  LOSS 

for  of  all  mammals  bred  in  captivity  the  mouse 
is  probably  richest  in  color  varieties.  In  one 
series  of  these  the  capacity  to  form  black  or 
brown  pigments  is  greatly  weakened,  so  that 
the  coat  is  less  heavily  pigmented  and  the  eye 
is  almost  wholly  unpigmented,  and  looks  pink, 
due  to  the  red  color  of  the  blood  in  the  eye. 
This  series  we  may  call  the  pink-eyed  series. 
All  the  common  color  varieties  occur  in  a  pink- 
eyed  as  well  as  in  a  dark-eyed  series.  Thus 
there  are  pink-eyed  grays,  pink-eyed  blacks, 
pink-eyed  cinnamons,  pink-eyed  browns,  and 
pink-eyed  yellows,  as  well  as  albinos  which 
transmit  the  pink-eyed  condition  in  crosses. 

Given  a  single  pink-eyed  individual  in  any 
one  of  these  varieties,  all  the  others  may  be 
produced  from  it  by  suitable  crosses.  Thus 
a  pink-eyed  gray  crossed  with  brown  produces 
in  F±  reversion  to  the  condition  of  the  wild 
house-mouse,  but  in  F2  (that  is,  among  the 
grandchildren)  occur  eight  varieties,  —  four 
dark-eyed  and  four  pink-eyed.  Gray,  black, 
cinnamon,  and  brown  occur,  both  in  dark-eyed 
and  in  pink-eyed  individuals,  the  latter  being 
also  far  lighter  in  color  than  the  dark-eyed 

79 


HEREDITY 

varieties.  The  pink-eyed  condition  is  there- 
fore in  mice  a  unit-character  modification  of 
the  pigmentation,  independent  of  any  of  the 
pigment  factors  previously  mentioned,  since 
it  can  be  transferred  by  crosses  from  asso- 
ciation with  one  of  these  to  association  with 
another.  It  may  also  be  transmitted  equally 
well  through  colored  and  through  albino  in- 
dividuals, though  it  produces  a  visible  effect 
only  in  colored  individuals. 

Another  unit-character  modification  of  the 
pigmentation  seen  in  mice  produces  a  series 
of  dilute  or  pale  pigmented  varieties,  but  dif- 
ferent in  character  from  the  pink-eyed  series, 
since  their  eyes  may  be  dark,  not  pink.  The 
pale  modification  of  gray  is  known  to  fan- 
ciers as  "  blue-gray, "  that  of  black  is  known 
as  "  blue,"  and  that  of  brown  is  known  as 
"  silver  fawn."  The  pale  quality  is  inter- 
changeable between  black,  brown,  and  yellow 
pigmentation,  so  that  if  one  has  a  pale  gray 
variety  he  may  by  crosses  obtain  also  pale 
black,  pale  cinnamon,  pale  brown,  and  pale 
yellow  varieties.  Or  if  one  starts  with  pale 
yellow,  he  may  by  crosses  with  a  perfectly 

80 


EVOLUTION   BY   LOSS 

wild  mouse  obtain  also  pale  gray,  pale  black, 
pale  cinnamon,  and  pale  brown  varieties,  all 
within  two  generations  from  the  cross. 

Now  the  pale  modification  is  distinct  from 
the  pink-eyed  modification,  and  independent  of 
it  in  transmission.  Accordingly,  it  is  possible 
to  have  the  two  modifications  combined  in  the 
same  race.  Thus  arises  a  series  of  pale  pink- 
eyed  grays,  blacks,  cinnamons,  browns,  and 
yellows.  Since  paleness  is  in  crosses  recessive 
to  intense  pigmentation,  and  pink  eyes  are  re- 
cessive to  dark  ones,  it  follows  that  a  variety 
which  is  both  pale  and  pink-eyed  will  breed 
true  to  those  characteristics  without  fixation. 

The  lightest  colored  of  the  pale  pink-eyed 
varieties  develop  very  little  pigment  indeed, 
yet  the  modifications  to  which  they  are  due 
are  wholly  different  in  nature  from  the  albino 
variation,  as  a  very  simple  experiment  will 
show.  Cross  together  an  albino  of  variety  (1), 
page  74,  —  which  is  a  snow-white  animal  with 
pink  eyes,  —  and  a  pale  pink-eyed,  brown  ani- 
mal, whose  coat  is  pale  straw  color,  and  whose 
eyes,  like  those  of  the  albino,  are  pink.  Although 
both  parents  are  pink-eyed,  and  one  develops  no 

81 


HEEEDITY 

pigment  whatever  in  its  fur,  while  the  other 
develops  very  little,  nevertheless  the  offspring 
are  as  dark  as  the  darkest  wild  mice,  eyes, 
fur,  and  all.  They  look  just  like  common  house- 
mice.  This  result  shows  that  the  albino  varia- 
tion is  something  very  different  in  nature  from 
the  modifications  found  in  the  pink-eyed  brown 
parent,  since  each  parent  contains  those  con- 
stituents of  the  wild  gray  coat  which  the  other 
parent  lacks. 

I  can  think  of  no  more  instructive  labora- 
tory experiment  illustrative  of  Mendelian  in- 
heritance than  to  follow  through  two  genera- 
tions the  cross  just  described,  and  to  analyze 
critically  the  results  obtained.  One  who  does 
this  can  never  be  sceptical  about  the  value  of 
crossing  as  an  agency  in  the  production  of  new 
varieties.  For  in  the  second  generation  from 
the  cross  he  will  obtain  (1)  ordinary  gray, 
black,  cinnamon,  and  brown  varieties;  (2)  pale 
gray,  black,  cinnamon,  and  brown  varieties; 
(3)  pink-eyed  gray,  black,  cinnamon,  and  brown 
varieties;  (4)  pink-eyed  and  pale  gray,  black, 
cinnamon,  and  brown  varieties;  and  lastly, 
albinos,  which,  if  he  has  the  patience  to  test 

82 


EVOLUTION   BY   LOSS 

them  one  by  one,  will  prove  to  be  of  sixteen 
different  homozygous  kinds,  to  say  nothing  of 
the  much  more  numerous  heterozygous  sorts. 

No  mention  has  thus  far  been  made  of  spotted 
races,  in  which  a  unit-character  modification  has 
occurred  which  results  in  a  distribution  of  pig- 
ment to  part  of  the  coat  only,  the  remainder 
being  unpigmented.  Although  this  modifica- 
tion apparently  regulates  the  distribution  of 
pigment  over  the  body,  it  is  independent  of  the 
general  color  factor  C,  since  it  is  transmitted 
through  albinos,  which  by  hypothesis  lack  C. 

Spotting  is  also  independent  of  all  the  other 
unit-character  modifications  which  have  been 
described.  Consequently  we  have  in  mice  four 
different  series  of  spotted  varieties,  —  the  in- 
tense spotted,  the  dilute  spotted,  the  intense 
pink-eyed  spotted,  and  the  dilute  pink-eyed 
spotted.  In  each  of  these  series  are  gray, 
black,  cinnamon,  brown,  and  yellow  individuals, 
making  a  total  of  twenty  spotted  sorts,  all  of 
which  may  be  obtained  from  crossing  a  single 
pair  of  properly  selected  parents,  such,  for 
example,  as  an  albino  and  a  wild  house-mouse 
of  the  kind  every  barn  contains. 

83 


HEREDITY 

The  color  variations  of  guinea-pigs  are  simi- 
lar to  those  of  mice;  the  same  series  of  unit- 
character  changes  has  produced  them  with  one 
exception.  The  pink-eyed  modification  is  want- 
ing in  guinea-pigs.  We  are  therefore  limited 
here  to  the  intense  series,  the  pale  series,  the 
intense  spotted  series,  and  the  pale  spotted 
series.  In  each  of  these  occur  gray  (or  agouti) 
individuals,  black  ones,  cinnamon  ones,  and 
brown  ones. 

The  parallelism  between  the  color  variations 
in  guinea-pigs  and  in  mice  received  an  inter- 
esting demonstration  in  a  particular  case.  The 
brown  pigmented  series  in  mice  has  been 
known  for  some  time,  but  in  guinea-pigs  the 
brown  variety  is  of  comparatively  recent  origin, 
and  the  cinnamon  variety  was  wholly  unknown 
until  some  three  years  ago.  After  an  analysis 
had  been  made  in  terms  of  unit-characters  of 
the  color  varieties  of  the  mouse,  it  became  clear 
that  if  the  color  variation  of  guinea-pigs  fol- 
lowed a  like  course,  a  then  unknown  variety 
of  guinea-pig,  cinnamon,  should  be  capable  of 
production  by  crossing  an  agouti  animal  with 
a  brown  one.  In  1907  a  statement  of  the  sci- 

84 


EVOLUTION   BY   LOSS 

entific  expectation  in  the  case  was  published, 
and  a  few  months  later  I  had  the  satisfaction 
of  announcing  its  fulfillment  in  the  second  gen- 
eration (F2)  from  the  cross  in  question. 

The  experiment  progressed  as  follows :  The 
parents  were  an  agouti  and  a  black,  their  Fx 
offspring  were  agoutis  in  character ;  but  the  F2 
offspring  were  of  four  sorts,  —  agouti,  black, 
cinnamon,  and  brown.  The  cross  thus  produced 
two  varieties  new  to  the  experiment,  viz.  black 
and  cinnamon,  the  latter  being  a  variety  at 
that  time  new  among  guinea-pigs. 

The  subsequent  behavior,  too,  of  the  newly 
produced  cinnamon  variety  is  in  harmony 
with  expectation  based  on  Mendelian  prin- 
ciples. The  cinnamon  variety  has  not  pro- 
duced agouti  or  black  individuals,  which  from 
the  formulae  it  will  be  seen  it  may  not  be  ex- 
pected to  produce,  since  it  lacks  the  factor  B. 
But  it  has  in  some  cases  produced  brown  in- 
dividuals, as  it  clearly  could  in  case  both  par- 
ents to  a  mating  were  heterozygous  (single) 
in  factor  A. 

On  the  whole  the  evidence  seems  very  clear 
that  the  numerous  color  varieties  of  animals 
7  85 


HEREDITY 

kept  in  captivity  arise  chiefly  from  loss  or 
modification  of  Mendelian  unit-characters. 
Loss  of  a  unit-character  might  easily  come 
about  by  an  irregular  cell-division  in  which 
the  material  basis  of  a  character  failed  to 
split,  as  normally.  The  consequence  would  be 
that  the  character  in  question  would  be  trans- 
mitted by  one  only  of  the  two  cell-products 
produced.  The  cell  lacking  a  character  might 
be  the  starting-point  of  a  race  lacking  the  char- 
acter, as  of  a  black  race,  derived  from  a  gray 
one.  On  the  other  hand  a  modified  condition 
of  a  unit-character  might  possibly  result  from 
unequal  division  of  the  material  basis  of  a 
character,  so  that  one  of  the  cell-products 
would  transmit  the  character  in  weakened  in- 
tensity, the  other  in  increased  intensity. 

BIBLIOGRAPHY 

CASTLE,  W.  E. 

1907.  "Color  Varieties  of  the  Rabbit  and  of  Other  Rod- 
ents:   Their  Origin  and  Inheritance."    Science,   N.  S., 
26,  pp.  287-291. 

1908.  "A  New  Color  Variety  of  the  Guinea-pig."   Science, 
N.  S.,  28,  pp.  250-252. 

1909.  "Studies   of  Inheritance  in  Rabbits."     Carnegie 
Institution  of  Washington,  Publication  No.  114,  70  pp., 
4  pi. 

86 


CHAPTER  VI 

EVOLUTION   OF    NEW   KACES  BY  VAKIATIONS   IN   THE 
POTENCY  OF  CHAEACTEKS 

IN  the  last  chapter  we  discussed  the  color 
variations  of  mammals,  and  we  concluded 
that  these  result  largely  from  the  loss  or 
modification  of  some  half-dozen  independent 
Mendelian  unit-characters.  As  to  the  material 
basis  of  these  unit-characters  some  interesting 
evidence  has  recently  been  collected  by  Riddle. 
Melanin  pigment  has  been  for  some  time  known 
to  be  formed  by  oxidation.  A  variety  of  or- 
ganic compounds  may  undergo  oxidation  into 
melanin  pigments  ranging  in  intensity  from 
light  yellow  to  black;  the  greater  the  oxida- 
tion, the  darker  the  product.  But  it  is  not 
certain,  as  assumed  by  Riddle,  that  the  chemi- 
cal method  of  oxidation  is  the  same  in  all  cases 
or  that  the  substance  to  be  oxidized  is  the  same. 
The  results  obtained  from  breeding  experiments 

87 


HEREDITY 

show  that  the  capacity  to  form  pigment  of  all 
sorts  may  be  lost  by  a  single  variation,  which 
we  have  called  loss  of  the  color  factor,  C.  We 
do  not  know  whether  it  consists  in  the  loss  of 
a  substance  capable  of  oxidation,  or  of  the 
power  to  take  some  indispensable  first  step  in 
the  process  of  oxidation,  perhaps  due  to  loss 
of  an  enzyme;  but  we  do  know  that  when  this 
particular  variation  has  occurred,  the  power 
to  produce  other  than  albino  individuals  can- 
not be  recovered  by  any  known  means  except 
a  cross  with  colored  animals.  We  know  also 
that  the  capacity  to  form  specific  kinds  of  pig- 
ment (yellow,  brown,  or  black)  is  independent 
of  the  general  color-factor,  C,  for  albinos  may 
transmit  those  specific  powers  without  them- 
selves being  able  to  form  any  kind  of  pigment 
at  all,  i.  e.  without  possessing  C.  Any  animal 
which  forms  pigment  of  one  of  the  higher 
grades  has  the  capacity  apparently  to  form 
pigment  also  of  the  lower  grades.  Thus  a 
black  animal  can  form  also  brown  and  yellow 
pigment  granules.  Brown  (chocolate)  animals, 
however,  lack  the  capacity  to  form  black  pig- 
ment. The  oxidation,  it  would  seem,  can  in 


VAEIATIONS    IN    POTENCY 

this  case  be  carried  no  further  than  the  brown 
stage,  because  of  the  lack  of  some  oxidizing 
agency  necessary  to  the  last  stage  in  pigment 
production.  The  production  of  yellow  is  prob- 
ably a  first  or  early  step  in  the  oxidation 
process  preliminary  to  the  production  of  brown 
or  black,  yet  all  yellow  animals,  so  far  as 
known,  are  able  to  take  the  further  steps ;  they 
retain  the  capacity  to  form  either  brown  or 
black  pigment  to  some  extent,  if  only  in  the 
eye. 

The  variations  thus  far  described  are  what 
De  Vries  has  called  retrogressive,  i.  e.  due  to 
loss  or  modification.  A  much  rarer  sort  of 
variation  has  been  called  by  De  Vries  progres- 
sive, i.  e.  due  to  gain,  acquisition  of  some  char- 
acter not  before  possessed  by  the  race.  I  can 
call  to  mind  very  few  cases  which  certainly 
fall  in  this  category.  One  which  it  would  seem 
must  belong  here  is  the  rough  or  resetted  con- 
dition of  the  hair  in  guinea-pigs,  a  variation 
similar  in  nature  to  the  reversed  plumage  of 
birds,  seen,  for  example,  in  the  Jacobin  pigeon. 
The  rough  coat  of  guinea-pigs  is  surely  not 
an  ancestral  condition,  yet  it  behaves  as  a 

89 


HEREDITY 

dominant  character  in  crosses.  It  can  scarcely 
be  explained  by  loss;  the  only  alternative  is 
to  consider  it  an  acquisition,  unless  we  choose 
to  consider  it  a  modification  of  the  normal 
condition. 

Aside  from  the  sorts  of  variations  already 
discussed,  which  consisted  either  in  the  loss  or 
modification  of  existing  unit-characters  or  in 
the  gain  of  new  ones,  we  must  also  recognize, 
as  a  cause  of  permanent  and  heritable  varia- 
tion, changes  in  the  potency  of  unit-characters, 
i.  e.  their  tendency  to  dominate  in  crosses. 

When  a  gamete  containing  a  particular  unit- 
character  unites  with  a  gamete  not  containing 
it,  the  zygote  formed  will  ordinarily  show  the 
character  in  question  fully  developed.  This  re- 
sult following  Mendel's  terminology  we  call 
dominance.  But  dominance  is  frequently  im- 
perfect and  may  even  be  reversed.  The  zygote 
in  which  a  character  is  doubly  represented  fre- 
quently develops  the  character  more  fully  than 
the  zygote  in  which  it  is  represented  but  once. 
If  a  black  guinea-pig  is  crossed  with  a  yellow 
one  the  offspring  are  black,  but  oftentimes  of 
a  slightly  yellowish  shade.  Likewise  if  black 

90 


VAKIATIONS    IN   POTENCY 

is  crossed  with  brown,  the  crossbreds  are  apt 
to  develop  in  their  coats  more  brown  pigment 
granules  than  do  homozygous  or  pure  blacks. 
Nevertheless,  we  have  no  reason  to  question  the 
entire  purity  of  the  gametes,  both  dominant 
arid  recessive,  formed  by  such  cross-bred  black 
animals.  It  is  the  dominance,  not  the  segrega- 
tion, which  is  imperfect. 

In  other  cases  still  the  dominance  may  be 
entirely  reversed  in  character,  owing  to  varia- 
tion in  the  potency  of  a  unit-character.  Thus 
in  most  rodents  the  gray  or  agouti  pattern- 
factor  of  the  hair,  A,  is  dominant.  A  cross  of 
black  with  homozygous  gray,  in  rats,  mice,  or 
rabbits,  produces  only  gray  offspring,  which 
in  F2  produce  three  grays  to  one  black.  But 
the  so-called  black  rat,  Mus  rattus,  a  species 
distinct  from  the  one  which  has  given  rise  to 
the  varieties  kept  in  captivity,  behaves  in  a 
different  way,  as  shown  by  Morgan  ('09). 
When  crossed  with  its  gray  variety,  the  roof 
rat,  Mus  alexandrinus ,  it  produces  only  black 
offspring,  and  in  F2,  three  blacks  to  one  gray. 
If  we  suppose  the  gray  coat  in  this  case  to 
be  due  to  the  same  factor  as  in  other  rodents, 

91 


HEEEDITY 

we  must  assign  to  it  a  different  potency,  or 
power  of  dominance,  so  that  it  produces  a 
visible  effect  only  when  doubly  represented  in 
the  zygote. 

In  guinea-pigs,  rabbits,  and  mice  we  have 
seen  that  the  presence  together  in  the  same 
zygote  of  two  factors,  A  and  B,  in  any  com- 
bination whatever,  produces  the  gray  or  agouti 
coat.  The  two  factors  are  A,  the  agouti  or 
gray  marking  of  the  hair,  and  B,  black  pig- 
ment in  the  fur.  If  A  is  lacking,  the  coat  is 
black;  if  B  is  lacking,  it  is  brown,  cinnamon, 
or  yellow.  If  both  are  lacking,  it  is  either 
brown  or  yellow.  But  if  both  are  present,  the 
wild  or  agouti  type  is  produced.  So  far  as 
the  production  of  the  agouti  coat  is  concerned, 
it  makes  no  difference  whether  either  factor  is 
singly  or  doubly  represented  in  the  zygote. 
Each  factor  has  potency  enough  to  produce  the 
full  effect  either  in  a  single  or  in  a  double 
dose.  Accordingly,  as  we  noticed  in  an  earlier 
chapter,  we  can  distinguish  by  their  breeding 
capacity,  though  not  by  their  looks,  four  types 
of  agouti  guinea-pigs  or  gray  rabbits,  viz.: 


VARIATIONS    IN   POTENCY 

1.  A  A  B  B,  which  breeds  true,  since  it  forms  game- 

tes all  AB; 

2.  ABB,     which  produces  agouti  young  and  black 

ones  in  the  ratio  3:1,  since  it  forms 
gametes  A  B  and  B  ; 

3.  A  A  B,     which  produces  agouti  young  and  yellow 

ones  in  the  ratio  3:1,  since  it  forms 
gametes  A  B  and  A; 

4.  A  B,         which  produces  agouti,  black,  and  yel- 

low young  in  the  ratio  9  :  3  :  4.  For 
the  gametes  formed  by  this  sort  are  of 
four  kinds,  A  B,  A,  B,  and  neither 
A  nor  B. 

Now  in  rats  we  have  no  evidence  that  the 
factor  B  has  ever  been  lost,  a  matter  to  which 
we  shall  presently  return ;  but  the  agouti  factor 
is  apparently  frequently  wanting  in  ordinary 
rats,  which  are  then  black.  For  ordinary  rats, 
then,  the  known  combinations  of  A  and  B  seem 
to  be  three,  viz. : 

A  A  B  B  =  the  pure  gray  (wild  type) ; 

ABB  =  heterozygous  gray,  which  produces  off- 
spring 3  gray  :  1  black.  This  type  is  ob- 
tained by  crossing  black  with  wild  gray; 

B  B  =  pure  black. 

Now  in  Mus  rattus,  as  we  have  seen,  the 
middle  or  heterozygous  type  is  black,  not  gray 

93 


HEREDITY 

in  appearance,  but  it  produces  both  the  gray 
and  the  black  types.  So  the  same  gametic 
formulae  will  account  for  both  sets  of  facts, 
if  we  suppose  merely  that  the  potency  of  A  is 
different  in  the  two  cases.  In  ordinary  rats 
(Mus  norvegicus)  A  produces  the  gray  coat  in 
a  single  dose;  but  in  Mus  rattus  its  potency 
is  less,  two  doses  are  required  to  produce  the 
gray  coat.  I  am  unable  to  frame  any  hypothe- 
sis other  than  this  which  will  account  for  the 
reversal  of  dominance  in  one  case  as  compared 
with  the  other. 

Yellow  color  in  mammals  affords  another 
illustration  of  this  same  thing,  —  reversal  of 
dominance.  Black  and  brown  are  in  most 
mammals  dominant  over  yellow  in  crosses,  but 
in  mice  the  reverse  is  true.  The  differential 
factor  between  black  and  yellow,  if  it  is  the 
same  in  mice  as  in  other  rodents,  must  be  in 
one  case  potent  enough  to  show  itself  if  singly 
represented  in  the  zygote,  whereas  in  the  other 
case  it  produces  no  visible  effect  unless  doubly 
represented  in  the  zygote.  Yellow  certainly 
seems  to  be  a  retrogressive  variation  from 
gray,  black,  or  brown.  The  pigment  granules 


VAKIATIONS    IN   POTENCY 

remain  in  a  lower  oxidation  stage  in  yellow  than 
in  black  or  brown.  We  suppose  that  in  the 
yellow  animal  something  is  wanting  which 
makes  that  further  oxidation  possible.  This 
hypothesis  would  fully  account  for  the  observed 
recessive  nature  of  yellow  in  the  case  of  all 
mammals  except  mice.  But  here  the  capacity 
to  form  black  or  brown  pigment  is  regularly 
present  in  the  yellow  individual  but  is  held  in 
check.  We  may  suppose,  therefore,  that  the 
differential  factor,  that  which  converts  yellow 
into  brown  or  black,  must  in  this  case  be  doubly 
represented  in  the  zygote  in  order  to  produce 
brown  or  black  fur,  whereas  in  most  mammals 
a  single  dose  is  effective.  Accordingly,  if  the 
unmodified  black  or  brown  factor  is  represented 
only  once  in  the  zygote,  and  the  yellow  modi- 
fication is  represented  once,  the  latter  will 
show,  since  the  former  is  singly  ineffective. 
The  animal  accordingly  is  a  heterozygous  yel- 
low, capable  of  producing  also  black  or  brown 
offspring.  But  mice  are  peculiar  in  that  they 
cannot  exist  in  the  doubly  deficient  condition 
of  a  pure  yellow  zygote,  consequently  all 
yellow  mice  are  heterozygous  dominants, 

95 


HEREDITY 

whereas   other  yellow  mammals  are  homozy- 
gous  recessives. 

In  connection  with  this  same  case  may  pos- 
sibly be  found  the  explanation  of  the  complete 
absence  of  the  yellow  variation  in  rats.  In 
nearly  all  mammals  kept  in  captivity  yellow  as 
well  as  black  varieties  occur;  this  is  true  of 
horses,  cattle,  swine,  dogs,  cats,  rabbits,  guinea- 
pigs,  and  mice.  In  rats,  however,  a  yellow 
variety  is  unknown.  We  know  that  rats  are 
able  to  form  yellow  pigment,  for  all  wild  rats 
do  form  yellow  pigment  in  their  agouti  fur,  yet 
singularly  enough  no  all-yellow  rat  has  ever 
been  observed,  so  far  as  we  have  any  record, 
either  wild  or  in  captivity.  A  rat  of  this  sort 
would  command  a  high  price  at  the  hands  of 
any  fancier.  Suppose  the  variation  did  occur 
in  a  single  gamete.  If,  as  in  most  mammals, 
it  behaved  as  a  recessive  in  crosses,  it  would 
not  become  visible,  and  might  be  carried  along 
for  untold  generations  without  ever  becoming 
visible  unless  two  yellow  gametes  met.  But  if, 
as  in  mice,  the  yellow-yellow  combination  when 
formed  quickly  perished,  then  the  character 
might  never  become  visible.  So  the  yellow 


VARIATIONS    IN   POTENCY 

variation  may  have  occurred  many  times  in 
rats,  as  it  has  in  so  many  other  mammals,  but 
failed  to  become  visible  simply  because  it  has 
the  same  potency  as  in  most  mammals,  but  is 
subject  to  the  same  physiological  limitations 
as  in  mice,  so  that  it  cannot  exist  in  a  homo- 
zygous  state.  In  that  case  the  only  evidence 
of  its  existence  in  a  race  would  lie  in  a 
slightly  diminished  fecundity  under  inbreeding, 
as  is  found  to  be  the  case  in  yellow  mice. 

Such  sharply  contrasted  variations  in  the 
potency  of  characters  as  we  have  been  discuss- 
ing are  evidently  of  prime  importance  in  evo- 
lution, making  all  the  difference  between  a 
dominant  and  a  recessive  condition  of  a  char- 
acter, or  between  the  occurrence  and  the  per- 
manent suppression  of  a  particular  variation. 
The  character  which  is  potent  enough  to  show 
itself  in  a  single  dose  will  behave  as  a  domi- 
nant character  in  crosses.  We  might  call  it 
ID/ i potent.  That  which  must  be  present  in  a 
double  dose  to  produce  a  visible  result  will 
behave  as  a  recessive  character  in  crosses.  We 
might  call  it  semi-potent.  It  is  not  impossible 
that  the  same  character  may  as  regards  domi- 

97 


HEREDITY 

nance  behave  in  different  ways  under  different 
circumstances,   at   one   time   dominating   com- 
•  pletely,  at  another  only  feebly,  and  at  other 
times  not  at  all. 

Undoubtedly  the  chief  condition  affecting 
dominance  is  the  nature  of  the  gamete  with 
which  a  union  is  made  in  fertilization.  In  1905 
(Carnegie  Inst.  Publ.  No.  23)  I  described  a 
case  in  which  a  particular  guinea-pig  (male 
2002,  shown  in  Fig.  32)  having  a  rough  or 
resetted  coat  gave  a  varying  result  in  crosses. 
In  crosses  with  most  smooth  animals  his  rough 
character  dominated  completely  (see  Fig.  24, 
which  shows  a  son  of  the  male  2002  by  a  smooth 
mother),  but  with  one  particular  smooth  ani- 
mal the  dominance  was  very  imperfect  in  all 
the  young  (Fig.  36),  while  with  a  second  it 
was  imperfect  in  half  the  young.  The  conclu- 
sion was  drawn  that  gametes  vary  in  potency, 
and  that  parents,  too,  differ  as  regards  the 
potency  of  the  gametes  which  they  produce, 
some  individuals  producing  gametes  all  of  which 
are  relatively  potent,  others  producing  gametes 
only  half  of  which  are  potent,  while  still  others 
produce  gametes  none  of  which  are  potent. 

98 


VARIATIONS    IN   POTENCY 

Relative  potency  would,  therefore,  seem  to  be 
a  character  inherited  in  Mendelian  fashion.1 

Observations  of  Coutagne  on  silk-moths  may 
be  cited  in  support  of  this  idea.  Coutagne 
made  crosses  between  races  of  silk-moths  dif- 
fering in  cocoon  color,  viz.  between  a  race  which 
spun  yellow  cocoons  and  another  one  which 
spun  white  cocoons.  He  found  that  some  of 
the  F!  offspring  spun  yellow  cocoons,  others 
white  ones.  The  F±  yellow  cocooned  animals 
when  bred  together  produced  F2  progeny  which 
spun  some  yellow,  others  white  cocoons,  the 
two  sorts  being  as  3:1.  In  other  words,  yel- 
low in  such  cases  behaved  consistently  as  a 
dominant  character.  And  the  white-cocooned 
F5  moths  produced  in  F2  cocoons  of  both  colors, 
but  in  this  case  the  white  cocoons  were  to  the 
yellow  ones  as  3:1.  In  other  words,  when 
yellow  behaved  as  a  dominant  in  Fx  it  behaved 
as  a  dominant  also  in  F2;  and  the  same  was 
true  of  white.  Each  retained  throughout  the 
two  generations  the  relative  potency  with  which 

1  It  is  of  course  possible  to  interpret  such  a  case  as  due  to  the 
separate  inheritance  of  a  factor  which  inhibits  the  development 
of  the  character,  but  it  is  doubtful  whether  this  line  of  explana- 
tion can  be  successfully  applied  to  cases  presently  to  be  described. 


HEEEDITY 

it  started.  C.  B.  Davenport  has  also  produced 
much  evidence  favoring  the  idea  of  varying 
potency  of  characters  in  recent  papers  based 
on  his  extensive  studies  on  poultry. 

The  case  which  I  described  in  1905  was  one 
in  which  unusual  potency  seemed  to  inhere  in 
the  gametes  of  a  recessive  individual,  —  one 
which  apparently  did  not  possess  the  character 
whose  dominance  was  affected.  But  there  occur 
also  cases  in  which  the  varying  gametic  potency 
is  associated  directly  with  the  character  af- 
fected. One  such  I  was  able  to  describe  in 
1906,  —  that  of  an  extra  toe  in  guinea-pigs.  It 
was  found  while  building  up  a  polydactylous 
race  by  selection  and  crossing  it  with  other 
races  that  individuals  varied  in  the  potency 
which  the  character  had  in  their  gametes.  In 
general  the  better  developed  the  character  was 
in  an  individual  the  more  strongly  was  it  trans- 
mitted, i.  e.  the  larger  was  the  proportion  of 
polydactylous  individuals  produced  in  crosses. 
In  no  case,  however,  was  this  a  recognizable 
Mendelian  proportion,  though  both  dominance 
and  segregation  seemed .  to  be  taking  place. 
Variation  in  potency  was,  however,  unmistak- 

,100, 


FIG.  36.  —  An  imperfectly  rough  guinea-pig.  Produced  by  mating 
the  guinea-pig,  shown  in  Fig.  32,  with  a  particular  smooth  animal; 
female,  2005. 

FIG.  37.  —  A  silvered  guinea-pig.  One  in  whose  coat  occur  white 
hairs  interspersed  with  pigmented  ones.  The  amount  of  the  silver- 
ing has  been  greatly  increased  by  selection. 

FIG.  38.  —  A.  Front  feet  of  an  ordinary  guinea-pig.  B.  Its  hind  feet. 
D.  Hind  feet  of  a  race  four-toed  on  all  the  feet.  C.  Ordinary  con- 
dition of  the  hind  feet  of  young  obtained  by  crossing  B  with  D. 

FIG.  39.  —  Diagram  showing  variation  in  the  color-pattern  of  hooded 
rats. 


VARIATIONS    IN   POTENCY 

able  and  was  transmitted  from  generation  to 
generation.1  See  Fig.  36. 

It  is  an  important  question  whether  potency 
is  a  property  of  the  unit-character  or  of  the 
gamete,  i.  e.  whether  it  affects  all  the  charac- 
ters transmitted  by  a  gamete  or  only  a  par- 
ticular one.  Practical  breeders  as  a  rule  favor 
the  idea  of  garnetic  rather  than  of  unit-character 
potency,  but  this  is  probably  due  to  a  failure 
to  discriminate  between  the  two.  They  desig- 
nate as  "  prepotent  "  an  individual  supposed 
to  impress  all  its  characters  upon  the  offspring, 
but  it  is  very  doubtful  whether  such  individuals 
exist.  It  is  easy  to  mistake  for  an  animal 
potent  in  all  respects  one  which  is  potent  in 
one  or  two  important  respects  only,  especially 
if  the  observer  is  unaware,  as  every  one  has 
been  until  quite  recently,  that  one  character  is 
independent  of  another  in  transmission. 

Conditions  other  than  the  character  of  the 
gametes  themselves  may  determine  the  extent 

1  An  alternative  explanation  is  possible,  viz.  that  the  develop- 
ment of  the  fourth  toe  depends  upon  the  inheritance  of  several 
independent  factors,  and  that  the  more  of  these  there  are  present, 
the  better  will  the  structure  be  developed.  The  correctness  of 
such  an  interpretation  must  be  tested  by  further  investigations. 
8  101 


HEEEDITY 

to  which  a  character  develops  in  the  zygote, 
i.  e.  the  completeness  or  incompleteness  of  its 
dominance  in  a  particular  case.  For  example, 
in  salamanders,  which  apparently,  like  mam- 
mals, form  skin-pigments  of  different  sorts, 
such  as  yellow,  brown,  and  black,  Tornier  has 
found  that  by  feeding  one  may  control  the 
proportions  in  which  chromatophores  of  the 
several  sorts  are  formed  in  the  skin.  Abundant 
feeding  causes  preponderance  of  pigment  of 
one  sort,  scanty  feeding  causes  preponderance 
of  pigment  of  another  sort.  Here  external 
conditions  determine  the  degree  of  development 
of  characters.  In  other  cases  internal  condi- 
tions may  exercise  a  controlling  influence.  Thus 
in  cattle  the  capacity  to  develop  horns  is  a 
semi-potent  unit-character,  behaving  as  a  re- 
cessive in  crosses,  heterozygotes  developing 
only  "  scurs,"  that  is,  mere  thickenings  of  the 
skin,  or  else  no  trace  of  horns  at  all.  In  sheep, 
moreover,  horns  are  more  strongly  developed 
in  males  than  in  females,  the  presence  of  the 
male  sex-gland  in  the  body,  or  rather  probably 
some  substance  given  off  into  the  blood  from 
the  sex-gland,  favoring  growth  of  the  horns. 

102 


VARIATIONS    IN    POTENCY 

In  merino  sheep  the  male  has  well-developed 
horns  but  the  female  is  hornless ;  yet  if  the  male 
is  castrated  early  in  life  no  horns  are  formed. 

When  a  breed  of  sheep  horned  in  both  sexes, 
such  as  the  Dorset,  is  crossed  with  one  horn- 
less in  both  sexes,  such  as  the  Shropshire,  horns 
are  borne  by  the  male  but  not  by  the  female 
offspring.  Both  sexes,  however,  are  heterozy- 
gous in  horns,  as  is  shown  by  their  breeding 
capacity.  For  in  F2  occur  both  horned  and 
hornless  individuals  in  both  sexes.  The  horn- 
less males  and  the  horned  females  prove  to 
be  homozygous,  but  the  horned  males  and  the 
hornless  females  may  be  either  heterozygous 
or  homozygous.  Accordingly  the  character, 
horns,  behaves  consistently  as  a  dominant  char- 
acter in  one  sex,  but  as  a  recessive  in  the 
other.  Further,  the  presence  of  the  male  sex- 
gland  in  the  heterozygote  raises  the  potency  of 
the  character,  horns,  from  semi-potent  to  uni- 
potent,  as  the  result  of  castration  shows. 

It  is  impossible  to  be  certain  that  in  a  horn- 
less race  the  character  horns  has  been  wholly 
lost.  It  may  merely  have  fallen  so  low  in 
potency  that  under  ordinary  conditions  it  pro- 

103 


HEREDITY 

duces  no  visible  structures.  The  occasional 
occurrence  of  an  imperfectly  horned  animal  as 
a  sport  within  a  hornless  race  need  not,  then, 
occasion  surprise.  It  would  be  a  variation  of 
the  same  sort  as  the  extra  toe  in  guinea-pigs 
(see  Fig.  38),  which,  from  a  single  sport,  was 
built  up  by  selection  into  a  well-established  race 
within  a  very  few  generations.  This  character, 
seemingly  lost  from  the  germ-plasm  for  an  in- 
definite period,  had  perhaps  merely  fallen  so  low 
in  potency  that  it  no  longer  produced  the  fourth 
toe  on  the  hind  foot,  though  this  was  still  pres- 
ent on  the  front  foot.  In  the  variant  observed, 
the  first  polydactylus  guinea-pig  of  my  stock, 
the  toe  was  imperfectly  developed  on  one  hind 
foot,  doubtless  as  the  result  of  an  unusually 
potent  condition  of  the  character  in  one  of  the 
gametes  which  produced  the  individual.  This 
manifestation  of  the  character,  though  feeble, 
was  sufficient  to  afford  a  guide  for  selection  of 
those  individuals  which  formed  the  most  potent 
gametes,  and  so  a  polydactylous  race  was 
formed  by  selection  and  inbreeding. 

Great  as  has  been  the  contribution  of  Men- 
delian  principles  to  our  knowledge  of  heredity, 

104 


VARIATIONS    IN   POTENCY 

they  do  not  reduce  the  whole  art  of  breeding 
to  the  production  of  new  combinations  of  unit 
characters  through  crossing.  Selection  is  re- 
quired also,  not  merely  among  different  combi- 
nations of  unit-characters,  but  also  among  in- 
dividuals representing  the  same  combinations 
selection  is  required  of  those  possessing  the 
desired  characters  in  greatest  potency.  The 
further  role  of  selection  in  evolution  we  shall 
need  to  consider  in  a  subsequent  chapter. 


BIBLIOGRAPHY 

CASTLE,  W.  E.,  and  LITTLE,  C.  C.  • 

1909.     "The  Peculiar  Inheritance  of  Pink  Eyes  Among 
Colored  Mice."    Science,  N.  S.,  30,  pp.  312-314. 

COUTAGNS,    G. 

1902.    "Recherches  expe*rimentales  sur  1'he're'dite'  chez  lea 

vers-a-soie."    Bull  Sci.,  37,  pp.  1-194,  9  pi. 
MORGAN,  T.  H. 

1909.      "Breeding   Experiments   with   Rats."   American 

Naturalist,  43,  pp.  182-185. 
DE  VRIES,  H. 

1901-03.    "Die  Mutationstheorie."   Leipzig,  Veit  and  Co. 

See  also  the  Bibliographies  to  Chapters  III.,  IV.,  and  V. 


CHAPTEE   VII 

CAN   MENDELIAN   UNIT-CHAKACTEKS  BE   MODIFIED 
BY  SELECTION? 

IF,  as  suggested  in  the  last  chapter,  the  po- 
tency of  a  character  in  crosses  may  be 
modified  by  selection,  why  may  not  the 
character  itself  be  modified  by  selection,  or  are 
not  the  two  things  perhaps  identical,  viz.  modi- 
fication of  the  potency  of  a  character  and  modi- 
fication of  the  character  itself?  Darwin  firmly 
believed  that  the  characters  of  organisms  can 
be  modified  by  selection,  and  he  made  this  the 
foundation  stone  of  his  theory  of  evolution. 
De  Vries  and  Johannsen,  however,  have  taught 
us  a  different  doctrine,  maintaining  that  selec- 
tion is  able  to  affect  characters  in  superficial 
and  transitory  ways  only,  that  the  slight  varia- 
tions in  characters  which  we  see  everywhere 
among  organisms  have  no  evolutionary  signifi- 
cance or  permanent  value ;  that  they  come  and 

106 


MENDELISM   AND    SELECTION 

go  like  the  wavelets  on  the  ocean  beach,  but 
have  no  more  relation  to  evolution  than  the 
waves  have  to  the  tides.  The  brilliancy  of  the 
Mutation  theory  of  De  Vries,  coupled  with  his 
great  service  to  biology  in  rediscovering  the 
Mendelian  laws,  has  somewhat  dazzled  our  eyes 
and  led  us,  I  think,  to  accept  too  readily  his 
views  concerning  the  efficacy  of  selection  also. 
Ten  years'  continuous  work  in  selection  con- 
vinces me  that  much  can  be  accomplished  by 
this  means  quite  apart  from  the  process  of  mu- 
tation. The  work  of  De  Vries  himself  argues 
strongly  in  favor  of  this  idea.  To  be  sure,  his 
interpretation  of  it  is  adverse  to  selection,  and 
has  seemed  to  most  of  us  at  times  overwhelm- 
ingly convincing;  but  from  his  interpretation 
we  may  fairly  appeal  to  the  record  of  the  work 
itself,  and  with  this  compare  the  record  of  our 
own  work. 

One  of  the  most  extensive  selection  experi- 
ments conducted  by  De  Vries  was  made  on  the 
common  buttercup,  Ranunculus  bulbosus,  which 
occurs  as  a  weed  in  pastures  and  meadows  in 
this  country  as  well  as  in  Europe.  It  has,  as 
is  known,  regular  5-petaled  flowers.  An  ex- 

107 


HEREDITY 

animation  of  717  flowers  in  the  field  made  by 
De  Vries  in  1887  showed  the  rather  frequent 
occurrence  of  6  and  7  petaled  flowers  also,  the 
average  number  of  petals  in  the  entire  collec- 
tion being  5.13.  De  Vries  set  himself  the  task 
to  see  if  the  proportion  of  many  petaled  flowers 
could  be  increased  or  the  number  of  petals  to 
a  flower  be  further  increased.  In  both  these 
respects  he  succeeded  surprisingly  well.  As  a 
result  of  five  successive  selections  the  average 
number  of  petals  was  raised  from  5.6  to  8.6, 
the  upper  limit  of  variation  from  8  to  31,  and 
the  mode  (or  commonest  condition)  from  5  to  9. 
Singularly  enough  De  Vries  concludes,  in  ac- 
cordance with  general  ideas  which  he  had 
adopted,  that  selection  had  in  this  case  done 
practically  all  that  it  could  accomplish,  that 
further  selection,  while  it  might  advance  the 
average  somewhat  farther,  would  have  no  per- 
manent effect  in  modifying  the  type.  This  be- 
lief seems  to  have  rested  on  considerations 
such  as  these.  De  Vries  had  found,  as  had 
others,  that  variations  which  are  heritable  have 
their  origin  in  the  germ-cells  only.  He  recog- 
nized that  the  tendency  to  produce  double 

103 


MENDELISM   AND    SELECTION 

flowers  in  the  buttercup  is  a  heritable  varia- 
tion and  supposed  it  to  be  a  unit-character, 
and  so  to  conform  with  Mendel's  law.. 

Now,  if  the  tendency  to  produce  double 
flowers  were  a  simple  Mendelian  character  it 
could  exist  in  only  three  conditions,  —  that  of 
a  recessive,  that  of  a  homozygous  dominant, 
or  that  of  a  heterozygous  dominant.  But  re- 
cessives  and  homozygous  dominants  are  pure, 
that  is,  they  form  only  one  type  of  gamete, 
and  selection  therefore  from  among  their  pro- 
geny could  produce  no  new  type.  As  regards 
the  heterozygous  dominant  type,  this  would 
itself  be  unfixable,  and  selection  could  accom- 
plish nothing  permanent  except  by  isolating  a 
homozygous  type.  But  such  types  should  all 
be  in  evidence  within  two  generations;  there- 
fore, if  a  completely  and  permanently  double 
type  had  not  been  discovered  within  the  five 
generations  covered  by  the  experiment,  such  a 
type  was  not  to  be  expected  at  all  from  the 
material  in  hand,  unless  either  a  wholly  new 
unit-character  were  introduced  or  an  existing 
one  were  profoundly  modified.  De  Vries  con- 
siders changes  of  both  these  sorts  possible. 

109 


HEEEDITY 

He  calls  them  mutations,  and  regards  them  as 
the  sole  means  of  evolutionary  progress.  But 
it  is  a  peculiarity  of  his  mutation  theory  that 
it  regards  only  large  changes  in  unit-characters 
as  having  any  permanency,  namely,  such 
changes  as  mean  a  practical  making  over  of 
the  character.  To  borrow  a  figure  from  Bate- 
son,  just  as  the  gas  carbon  monoxide,  C  0, 
may  change  into  a  very  different  gas,  —  carbon 
dioxide,  C  02,  —  by  taking  up  a  single  atom 
of  oxygen,  but  can  make  no  less  extensive 
change,  since  oxygen  atoms  do  not  split;  so, 
according  to  De  Vries,  a  unit-character  may 
not  change  unless  it  changes  profoundly.  Vari- 
ous circumstances  may  modify  the  degree  of 
its  expression,  but  these  are  without  perma- 
nent effect,  since  the  character  itself  remains 
unchanged. 

But  there  are  both  a  priori  and  experimental 
grounds  for  questioning  the  correctness  of 
De  Vries '  conclusions.  It  is  known  that  the 
chemical  compounds  within  the  germ-cells  are 
not  so  simple  in  composition  as  C  0  and  C  02. 
They  are  very  complex  substances,  made  up, 
it  is  thought,  of  very  many  atoms,  often  hun- 

110 


MENDELISM  AND  SELECTION 

dreds  in  a  single  molecule.  If  so,  it  is  quite 
possible  that  an  atom  or  two  might  be  trans- 
posed in  position  within  the  molecule  without 
wholly  altering  its  chemical  nature,  and  that 
thus  slight  changes  in  the  germ-plasm  might 
result,  which,  however,  would  be  as  permanent 
as  more  profound  changes. 

The  argument  of  De  Vries  against  any  per- 
manent effect  of  selection  in  modifying  unit- 
characters  has  been  greatly  strengthened  by 
the  subsequent  work  of  Johannsen  and  Jennings. 
Johannsen  has  found  that  if  one  selects  from  a 
handful  of  ordinary  beans  the  largest  seeds  and 
the  smallest  seeds,  and  plants  these  separately, 
the  former  will  produce  beans  of  larger  average 
size  than  the  latter.  Selection  here  has  effect. 

But  if  the  selection  is  made,  not  from  a 
general  field  crop  of  beans,  but  from  those 
beans  borne  on  one  and  the  same  homozygous 
mother  plant,  then  the  progeny  of  the  selected 
large  seed  will  be  no  larger  than  that  of  the 
selected  small  seed.  Selection  here  is  without 
effect. 

The  different  result  in  the  two  cases  may 
be  explained,  according  to  Johannsen,  on 

111 


105 


FIG.  40.  —  Diagram  showing  the  variations  in  size  of  eight  different  races  of 
paramecium.  Each  horizontal  row  represents  a  race  derived  from  a 
single  parent  individual.  The  individual  showing  the  mean  size  in 
each  race  is  indicated  by  a  cross  placed  above  it.  The  mean  of  the 
entire  lot  is  shown  at  X  —  X.  The  numbers  show  the  measure- 
ments in  microns.  (After  Jennings.) 

112 


MENDELISM   AND    SELECTION 

the  principle  of  the  "  pure  line."  The  pro- 
geny of  a  single  self-fertilized  homozygous 
bean  plant  constitute  a  pure  line.  They  are 
all  alike,  so  far  as  the  hereditary  transmission 
of  size  is  concerned,  for  they  are  all  derived 
from  like  gametes.  The  differences  in  size 
which  occur  among  them  are  due  to  differences 
in  nutrition,  not  to  germinal  differences,  and 
they  are  not  transmitted.  But  in  a  mixed 
population  of  beans,  such  as  is  represented  by 
a  field  crop,  differences  of  size  occur  which 
are  due  to  heredity  as  well  as  those  which  are 
due  to  the  environment.  In  the  case  of  the 
former,  selection  naturally  has  effect;  in  the 
case  of  the  latter,  it  does  not. 

Jennings  has  obtained  similar  results  in  his 
studies  of  paramecium,  —  a  one-celled  animal 
which  multiplies  asexually  by  dividing  into  two 
similar  parts.  It  lives  in  stagnant  water  and 
may  be  reared  in  great  numbers  in  a  hay-infu- 
sion, for  it  multiplies  with  great  rapidity,  divid- 
ing two  or  three  times  within  twenty-four  hours. 
The  variations  in  size  which  occur  in  parame- 
cium are  shown  in  Fig.  40. 

When  from  an  ordinary  culture  of  parame- 
113 


HEREDITY 

cium  Jennings  selects  the  largest  and  the  small- 
est individuals  respectively,  he  finds  that  the 
descendants  of  the  one  lot  will  be  of  larger  size 
than  the  other.  This  looks  like  an  effect  of  selec- 
tion upon  racial  size.  But  if  selection  is  made  not 
within  a  mixed  population  but  among  the  de- 
scendants of  a  single  individual,  it  is  found 
that  the  descendants  of  large  individuals  are 
of  no  greater  average  size  than  those  of  small 
individuals. 

The  explanation  of  this  fact  is  to  be 
found  in  the  existence  of  what  Johannsen 
has  called  pure  lines.  Jennings  has  been  able 
to  isolate  eight  distinct  pure  lines  of  parame- 
cium  differing  in  average  size,  as  shown  in 
Fig.  40.  The  range  of  variation  in  size  within 
one  of  these  races  is  great,  but  if  one  selects 
extremely  large  or  extremely  small  individuals 
within  the  same  pure  line,  i.  e.  among  the 
asexually  produced  descendants  of  the  same 
animal,  no  change  in  the  average  size  of  the 
race  is  brought  about. 

A  very  different  result  is  obtained,  however, 
if  one  mixes  together  several  pure  lines  and  then 
selects  from  the  mixed  race  on  the  basis  of  size. 

114 


MEKDEUSM   AND    SELECTION 

The  larger  animals  then  produce  larger  average 
offspring  and  vice  versa.  An  examination  of 
Fig.  40  will  show  why.  Animals  of  the  same  ab- 
solute size  are  there  placed  in  the  same  vertical 
row.  If,  now,  one  selects  from  the  mixed  popu- 
lation only  the  largest  individuals,  he  will  nat- 
urally secure  representatives  of  only  two  or 
three  pure  lines,  viz.  of  those  lines  which  are 
characterized  by  the  largest  average  size,  and 
which,  therefore,  will  produce  large  average 
offspring.  If  on  the  other  hand  he  selects 
extremely  small  individuals,  he  will  secure 
representatives  of  only  the  smallest  races, 
which  naturally  will  produce  small  offspring, 
so  that  selection  seems  to  be  effective  in  modi- 
fying racial  size,  but  in  reality  it  does  this  by 
sorting  out  the  elementary  constituents  of  the 
race. 

It  is  impossible  to  deny  the  soundness  of 
the  reasoning  of  Johannsen  and  Jennings.  It 
is  perfectly  clear  that  the  effects  of  selection 
should  be  more  immediate  and  much  greater 
in  the  case  of  a  mixed  race  than  in  that  of  a 
pure  line,  but  is  it  certain,  as  assumed  by 
them,  that  selection  is  wholly  without  effect  in 

115 


HEREDITY 

the  case  of  a  pure  line?  We  know  the  effects 
should  be  less,  but  are  they  nil?  Concerning 
this  matter  we  are  perhaps  justified  in  await- 
ing further  evidence.  For  in  the  case  of  beans 
and  of  paramecium  alike  size  is  subject  to  very 
great  variation  through  the  influence  of  nutri- 
tion. Variations  due  to  this  cause  are  natu- 
rally not  inherited,  since  the  germ-cells  are  not 
affected  by  them,  but  only  the  body.  But  is 
it  not  possible  that  along  with  the  striking  size 
differences  due  to  nutrition  there  may  occur 
also  slight  size  differences  due  to  germinal 
variation  within  the  pure  line,  that  is  owing 
to  variations  in  the  potency  of  the  same  unit- 
character  or  combination  of  unit-characters? 
To  be  sure,  Johannsen  and  Jennings  have  not 
observed  these,  but  this  does  not  prove  their 
non-existence.  Others  may  yet  be  able  to  do 
so;  indeed  one  case  is  already  on  record  in 
which  such  observations  have  been  made  in  the 
case  of  a  small  crustacean  (or  water-flea), 
Daphnia. 

Daphnia  is  a  small  transparent  animal,  about 
the  size  of  a  pin-head,  which  occurs  in  enor- 
mous numbers  in  fresh-water  lakes  and  pools, 

116 


MENDELISM   AND    SELECTION 

forming  a  large  part  of  the  food  supply  of 
fresh-water  fishes.  It  multiplies  chiefly  by  the 
production  of  unfertilized  eggs,  —  those  which 
undergo  no  reduction  and  which  develop  with- 
out fertilization  into  an  individual  like  the 
parent.  The  germinal  composition,  therefore^ 
of  all  descendants  produced  in  this  way  by  the 
same  mother  should  be  identical,  unless  germi- 
nal composition  can  be  modified  in  other  ways 
than  by  reduction  and  recombination  of  unit- 
characters.  Now  the  German  zoblogist,  Wol- 
tereck,  has  shown  that,  among  the  offspring 
developed  from  the  unfertilized  eggs  of  the 
same  mother  Daphnia,  variations  do  occur 
which  are  heritable,  so  that  if  one  selects  ex- 
treme variants  he  obtains  a  modified  race. 
Systematic  zoologists  recognize  as  a  generic 
distinction  between  Daphnia  and  Hyalodaphnia 
absence  from  the  latter  of  the  rudimentary  eye 
found  in  Daphnia.  Woltereck  observed  that 
in  a  pure  line  of  Hyalodaphnia  the  rudi- 
mentary eye,  usually  wanting,  may  occur  in 
individual  cases.  He  found  further  that  it 
occurred  in  varying  degrees  of  development, 
which  ranged  all  the  way  from  a  group  of 
9  117 


HEREDITY 

pigmented  cells  outside  the  brain,  through 
stages  in  which  cells  were  present  without  pig- 
ment, and  others  in  which  pigment  was  visible 
within  the  brain  but  no  cells  outside  it  were 
developed,  and  finally  to  those  in  which  all 
traces  of  the  eye  had  vanished,  cells  and  pig- 
ment alike.  By  selection  in  three  successive 
generations  of  the  mother  having  the  rudi- 
mentary eye  best  developed  offspring  were 
obtained,  90  %  of  which  had  the  pigmented 
eye,  and  which  would  therefore  pass  for  ani- 
mals of  a  wholly  different  genus.  The  degree 
of  development  of  the  organ  in  the  last  genera- 
tion was  also  greater  than  in  the  previous 
generations.  Here  within  a  pure  line  produced 
by  parthenogenesis  selection  served  to  augment 
both  the  degree  of  development  of  an  organ 
and  the  frequency  of  its  occurrence  within  the 
race,  a  result  precisely  parallel  to  that  which 
I  obtained  some  years  ago  by  selection  in  the 
case  of  a  rudimentary  fourth  toe  in  the  guinea- 
pig.  The  experiment  with  Daphnia  is  not  open 
to  the  objection  that  may  be  offered  to  the 
guinea-pig  experiment,  that  it  is  possibly  a 
result  of  gametic  segregation  and  recombina- 

118 


MENDELISM   AND    SELECTION 

tion,  for  in  Daphnia  the  reproduction  was  ex- 
clusively by  unreduced  and  unfertilized  eggs. 

The  rudimentary  eye  of  Daphnia  is  an  organ 
the  development  of  which,  so  far  as  observed, 
is  wholly  independent  of  environmental  influ- 
ence; but  the  case  is  different  with  another 
structure  of  Daphnia,  upon  which  also  Wol- 
tereck  made  observations,  namely,  a  projection 
or  spine  borne  on  the  head  of  the  animal. 
This  is  not  a  constant  structure,  but  is  some- 
times present,  sometimes  wanting  altogether,  in 
the  same  pure  line.  In  extreme  cases  it  forms 
a  great  angular  extension  of  the  head  forward. 
To  a  considerable  extent  its  development  is 
subject  to  control  through  the  temperature  of 
the  surrounding  water,  but  independently  of 
such  influence  the  degree  of  its  development 
varies  and  is  heritable.  Although  in  general, 
just  as  in  the  experiments  of  Johannsen  and 
Jennings,  selection  of  animals  with  the  best- 
developed  spine  did  not  increase  the  degree  of 
development  of  the  organ  or  the  frequency  of 
its  occurrence,  yet  in  individual  cases  such 
increase  was  observed,  so  that  the  structure 
occurred  in  over  50  %  of  the  offspring.  In 

119 


HEEEDITY 

such  cases,  then,  it  would  seem  that  along  with 
the  cases  due  to  environmental  influence  oc- 
curred others  due  to  germinal  variation.  Al- 
though selection  of  the  former  would  not  in- 
fluence the  race  permanently,  there  is  every 
reason  to  think  that  the  latter  would  so  influ- 
ence it,  and  did  in  the  experiment. 

Accordingly  the  results  of  Johannsen  and 
Jennings  on  the  one  hand,  and  of  Woltereck 
on  the  other,  are  not  necessarily  in  opposition 
to  each  other.  Woltereck 's  conclusions  agree 
with  those  of  Johannsen  and  Jennings  so  far 
as  concerns  the  great  bulk  of  the  variations, 
those  caused  by  external  influences.  All  agree 
that  they  are  not  inherited.  Woltereck,  how- 
ever, observes  also,  what  the  others  have  failed 
to  observe,  that  along  with  the  non-inherited 
variations  occur  other  similar  but  less  numer- 
ous ones  which  are  inherited. 

My  own  observations  are  entirely  in  har- 
mony with  those  of  Woltereck.  Like  him,  I 
find  that  selection  may  modify  characters.  In 
several  cases  I  have  observed  characters  at 
first  feebly  manifested  gradually  improve  under 
selection  until  they  became  established  racial 

120 


MENDELISM   AND    SELECTION 

traits.  Thus  the  extra  toe  of  polydactylous 
guinea-pigs  made  its  appearance  as  a  poorly 
developed  fourth  toe  on  the  left  foot  only. 
Only  6  %  of  the  offspring  of  this  animal  by 
normal  unrelated  mothers  were  polydactylous, 
but  among  his  offspring  were  some  with  better 
developed  fourth  toes  than  the  father  pos- 
sessed. Such  individuals  were  selected  through- 
out five  successive  generations,  at  the  end  of 
wliich  time  a  good  four-toed  race  had  been 
established.  It  was  found  in  general  that  those 
animals  which  had  best-developed  fourth  toes 
transmitted  the  character  most  strongly  in 
crosses  with  unrelated  normal  animals.  The 
percentage  of  polydactylous  individuals  pro- 
duced in  such  crosses  varied  all  the  way  from 
0  to  100  %.  By  selection  this  percentage  was 
increased,  as  was  also  the  degree  of  develop- 
ment of  the  fourth  toe  in  crosses. 

Another  character  which  made  its  appear- 
ance among  our  guinea-pigs,,  at  first  feebly 
expressed,  was  a  silvering  of  the  colored  fur, 
due  to  interspersing  of  white  hairs  with  the 
colored  ones  (see  Fig.  37).  The  first  indi- 
viduals observed  to  have  this  character  bore 

121 


HEREDITY 


white  hairs  on  the  under  surface  of  the  body 
only.  By  inbreeding,  a  homozygous  strain  of 
the  silvered  animals  was  soon  obtained,  one  in 


43 


+  2 


-2 


gen 

FIG.  41.  —  Chart  showing  effects  of  selection  in  eight  successive 
generations  upon  the  color-pattern  of  hooded  rats.  A, 
average  condition  of  the  selected  parents  in  the  plus  series; 
B,  average  condition  of  their  offspring.  A1,  average  condi- 
tion of  the  selected  parents  in  the  minus  series;  B1,  average 
condition  of  their  offspring. 

which  all  the  offspring  were  silvered  to  a 
greater  or  less  extent.  Selection  was  now 
directed  toward  two  ends,  —  (1)  to  secure  ani- 
mals which  were  free  from  spots  of  red  or 
white,  a  condition  which  was  present  in  the 

122 


MENDELISM    AND    SELECTION 

original  stock,  and  (2)  to  secure  extensive  and 
uniform  silvering  on  a  black  background.  In 
both  these  objects  good  progress  has  been 
made.  We  have  animals  which  are  silvered  all 
over  the  body  except  on  a  part  of  the  head, 
and  the  percentage  of  such  well-silvered  indi- 
viduals is  relatively  high. 

But  the  most  extensive  selection  experiment 
which  I  have  personally  observed  is  one  in 
which  I  have  been  assisted  by  Dr.  John  C. 
Phillips  (see  Figs.  39  and  41).  Selection  in 
this  case  has  been  directed  toward  a  modifica- 
tion of  the  color  pattern  of  hooded  rats,  —  a 
pattern  which  is  known  to  behave  as  a  reces- 
sive Mendelian  character  in  crosses  with  either 
the  self  (totally  pigmented)  condition  or  the 
so-called  Irish  (white-bellied)  condition  found 
in  some  other  rats.  The  extreme  range  of 
variation  among  our  hooded  rats  at  the  outset 
of  this  experiment  is  indicated  by  the  grades 
-  2  and  +  3  of  Fig.  39.  Selection  was  now 
made  of  the  extreme  variates  in  either  direc- 
tion and  these  were  bred  separately.  Two 
series  of  animals  were  thus  established,  —  one 
of  narrow  striped  animals,  minus  series;  the 

123 


HEREDITY 

other  of  wide  striped,  plus  series.  In  each 
generation  the  most  extreme  individuals  were 
selected  as  parents ;  in  the  narrow  series,  those 
with  narrowest  stripe ;  in  the  wide  series,  those 
with  widest  stripe. 


TABLE   I 

Results  of  Selection  for  Modification  of  the  Color-pattern 
of  Hooded  Rats. 


Plus  series. 


Minus  series. 


GENERA- 
TION. 

1 
2 
3 

4 
5 
6 

7 
8 


AVERAGE 
GRADE, 

PARENTS. 

2.50 
2.51 
2.73 
3.09 
3.33 
3.51 
3.53 
3.65 


1.46 
1.41 
1.56 
1.69 
1.73 
1.86 
2.00 
2.03 


AVERAGE        NUMBER 
GRADE,         OF  OFF- 
OFFSPRING.        SPRING. 


2.05 

150 

1.92 

471 

2.51 

341 

2.72 

444 

2.90 

610 

3.09 

834 

3.14 

874 

3.30 

91 

3,815 

1.00 
1.07 
1.18 
1.28 
1.41 
1.56 
1.70 
1.78 


55 

132 

195 

329 

701 

1252 

1544 

713 

4,921 


MENDELISM   AND    SELECTION 

The  result  of  the  selection  is  shown  graph- 
ically in  Fig.  41  (compare  Table  I).  The 
offspring  in  the  narrow  series  became  with 
each  generation  narrower;  those  in  the  wide 
series  became  with  each  generation  wider,  with 
a  single  exception.  In  generation  two  the  wide 
stock  was  enlarged  by  the  addition  of  a  new 
strain  of  animals.  This  caused  a  temporary 
falling  off  in  the  average  grade  of  the  young, 
the  two  series  overlapping  for  that  generation. 
No  new  stock  was  at  any  other  time  intro- 
duced in  either  series,  the  two  remaining  dis- 
tinct at  all  times  except  in  generation  two. 
It  will  be  observed  that  a  change  in  the  aver- 
age grade  of  the  parents  is  attended  by  a 
corresponding  change  in  the  average  grade 
of  the  offspring.  The  amount  of  variabil- 
ity of  the  offspring  is  not  materially  affected 
by  the  selection,  but  the  average  about  which 
variation  occurs  is  steadily  changed,  as  are 
also  the  limits  of  the  range  of  variation. 

The  interesting  feature  of  this  experiment 
is  the  production,  as  a  result  of  selection,  of 
wholly  new  grades ;  in  the  narrow  series,  of  ani- 
mals having  less  pigment  than  any  known  type 

125 


HEREDITY 

other  than  the  albino;  in  the  wide  series,  of 
animals  so  extensively  pigmented  that  they 
would  readily  pass  for  the  "  Irish  type,"  which 
has  white  on  the  belly  only,  but  which  is  known 
to  be  in  crosses  a  Mendelian  alternative  to  the 
hooded  type.  By  selection  we  have  practically 
obliterated  the  gap  which  originally  separated 
these  types,  though  selected  animals  still  give 
regression  toward  the  respective  types  from 
which  they  came.  But  this  regression  grows 
less  with  each  successive  selection  and  ulti- 
mately should  vanish,  if  the  story  told  by  these 
statistics  is  to  be  trusted.  As  yet  there  is  no 
indication  that  a  limit  to  the  effects  of  selec- 
tion has  been  reached. 

From  the  evidence  in  hand  we  conclude  that 
Darwin  was  right  in  assigning  great  importance 
to  selection  in  evolution;  that  progress  results 
not  merely  from  sorting  out  particular  com- 
binations of  large  and  striking  unit-characters, 
but  also  from  the  selection  of  slight  differences 
in  the  potentiality  of  gametes  representing  the 
same  unit-character  combinations.  It  is  pos- 
sible to  ascribe  such  differences  to  little  units 
additional  to  the  recognized  larger  ones,  but 

126 


MENDELISM   AND    SELECTION 

if  such  little  units  exist,  they  are  indeed  very 
little  as  well  as  numerous,  and  by  adding  to  the 
effect  of  the  larger  ones  they  produce  what 
amounts  to  modification  of  them. 


BIBLIOGRAPHY 

CASTLE,  W.  E. 

1906.     "The  Origin  of  a  Polydactylous  Race  of  Guinea- 
pigs."     Carnegie  Institution  of  Washington,  Publication 
No.  49,  pp.  17-29. 
JENNINGS,  H.  S. 

1909.  "  Heredity  and  Variation  in  the  Simplest  Organ- 
isms." The  American  Naturalist,  43,  pp.  321-337. 

JOHANNSEN,    W. 

1909.     "Elemente   der  exakten  Erblichkeitslehre."     G. 

Fisher,  Jena,  516  pp. 
DE  VRIES,  H. 

(See  Bibliography  to  Chapter  VI.) 

WOLTERECK,    R. 

1909.  "  Weitere  experimentelle  Untersuchungen  iiber  Art- 
veranderungen,  speciell  tiber  das  Wesen  quantitativer 
Artunterschiede  bei  Daphniden."  Verh.  Deutsch.  Zool. 
Gesellsch.,  pp.  110-172. 


CHAPTER   VIII 

MENDELIAN  INHERITANCE   WITHOUT  DOMINANCE, 
"  BLENDING  "  INHERITANCE 

WE    shall   now   discuss   a   seemingly 
different  type  of  inheritance  from 
that  discovered  by  Mendel,  —  one 
in  which  the  offspring  are  a  true  intermediate 
or  blend  between  the  parents,  and  in  which  the 
occurrence  of  segregation  has  not  in  all  cases 
been  certainly  established. 

Differences  in  size  between  parents  have  been 
found  to  behave  in  this  blending  fashion.  Rabbits 
are  apparently  favorable  material  in  which  to 
study  size  inheritance,  for  some  races  are  fully 
twice  as  large  as  others.  If  a  large  rabbit  is 
crossed  with  a  small  one  the  young  are  of  inter- 
mediate size,  and  the  F2  offspring  show  no  such 
segregation  into  large,  small,  and  intermediate 
individuals  as  a  simple  Mendelian  system  would 
demand.  For  this  reason  size  has  been  de- 

128 


FIG.  42.  — Skulls  of   three   rabbits.     Father  (1    and    la), 
mother  (3  and  3a),  and  son  (2  and  2a). 


"BLENDING"    INHEKITANCE 

scribed  as  a  non-Mendelian,  non-segregating 
type  of  inheritance,  but  recent  discoveries  place 
this  interpretation  in  doubt.  Let  us  first  con- 
sider what  are  the  observed  facts  and  after- 
ward the  interpretation. 

Fig.  42  shows  the  skulls  of  three  rabbits,  - 
of  the  father  at  the  left,  of  the  mother  at  the 
right,  and  that  of  the  son  between.  Notice  the 
fully  intermediate  or  blended  character  of  the 
son's  skull  as  regards  both  absolute  dimensions 
and  proportions.  The  intermediate  character 
was  possessed  also  by  the  next  generation  of 
offspring.  Now  this  same  cross,  while  pro- 
ducing a  blend  in  size  and  ear-length,  was 
yielding  dominance  and  segregation  of  coat- 
characters.  Fig.  43  shows  a  picture  of  the 
rabbit  with  the  small  skull  in  the  cross  just 
described.  He  was  an  albino  and  his  fur  was 
long.  The  mother,  which  had  the  large  skull, 
was  a  sooty-yellow  rabbit,  with  short  fur  and 
long  ears  (see  Fig.  44).  The  son  is  shown 
in  Fig.  45.  His  fur  was  black  and  short,  the 
albinism  and  long  fur  of  his  father  having 
become  recessive  in  the  cross  in  accordance 
with  Mendel's  law.  The  pigmentation  is  also 

129 


HEREDITY 

intensified  in  the  son,  black  having  been  re- 
ceived through  the  albino  parent  as  a  latent 
factor,  which  became  fully  active  in  the  son. 
The  excluded  albinism,  recessive  in  the  son  and 
his  brothers  and  sisters,  all  seven  of  which 
were  similar  in  character,  reappeared  among 
the  grandchildren,  as,  for  example,  in  the  one 
shown  in  Fig.  46,  which  was  short-haired. 
Other  F2  offspring  were  long-haired,  some  of 
them  being  albinos,  others  being  pigmented. 
But  the  size  and  ear-length  of  the  son  were 
intermediate  between  the  sizes  and  ear-lengths 
of  his  parents,  and  this  intermediate  character 
persisted  without  apparent  segregation  among 
the  F2  offspring.  The  animals  in  the  pictures 
are  unfortunately  not  all  shown  on  the  same 
scale,  but  the  relative  ear-lengths  are  sufficiently 
clear. 

A  Mendelian  interpretation  of  blending  in- 
heritance, illustrated  in  the  inheritance  of  skull- 
size  and  ear-length  among  rabbits,  has  been  sug- 
gested by  my  colleague  Dr.  East,  and  by  others, 
an  interpretation  in  which  Mendelian  dominance 
is  indeed  wanting  but  segregation  nevertheless 
occurs,  yet  not  of  a  simple  kind,  involving  one 

130 


" BLENDING"    INHEKITANCE 

or  two  segregating  factors,  but  involving  sev- 
eral such  factors.  Before  entering  into  this  ex- 
planation it  will  be  necessary  to  discuss  a  fur- 
ther extension  of  Mendelian  principles  recently 
made. 

Some  modified  Mendelian  ratios  of  particular 
interest  have  lately  been  obtained  by  the  Swedish 
plant-breeder,  Nilsson-Ehle  (1909,  Lunds  Uni- 
versitets  Arsskrift)  in  crossing  varieties  of 
wheat  of  different  color.  When  a  variety  hav- 
ing brown  chaff  is  crossed  with  one  which  has 
white  chaff,  the  hybrid  plants  are  regularly 
brown  in  F±  and  3  brown  :  1  white  in  F2,  but 
a  particular  variety  of  brown-chaffed  wheat 
gave  a  different  result.  In  15  different  crosses 
it  gave  uniformly  a  close  approximation  to 
the  ratio  15  : 1  instead  of  3  : 1.  The  totals  are 
sufficiently  large  to  leave  no  doubt  of  this. 
They  are  1410  brown  to  94  white,  exactly  15  : 1. 
This  is  clearly  a  dihybrid  Mendelian  ratio,  and 
Nilsson-Ehle  interprets  it  to  mean  that  there 
exist  in  this  case  two  independent  factors,  each 
of  which  is  able  by  itself  to  produce  the  brown 
coloration,  though  no  qualitative  difference  can 
be  detected  between  them. 

131 


HEEEDITY 

A  still  more  remarkable  case  was  observed 
in  crosses  between  varieties  of  wheat  of  dif- 
ferent grain-color.  Eed  crossed  with  white 
gave  ordinarily  all  red  in  FA  and  3  red  to  1 
white  in  F2,  but  a  certain  native  Swedish  sort 
gave  only  red  (several  hundred  seeds)  in  F2. 
This  result  was  so  surprising  that  one  cross 
which  had  yielded  78  grains  of  wheat  in  F2  was 
followed  into  F3,  with  the  following  result: 

50  plants  gave  only  red  seed;  expected  37 

5  "  "  approximately  63  R:  1  W;  "  8 

15  "  15R:1W;  "  12 

8  "  "  "  3R:1W;  "  6 

0  "  all  white;  1 

The  interpretation  given  by  Nilsson-Ehle  is 
this.  The  red  variety  used  in  this  cross  bears 
three  independent  factors,  each  of  which  by 
itself  is  able  to  produce  the  red  character. 
Their  joint  action  is  not  different  in  kind  from 
their  action  separately,  though  possibly  quanti- 
tatively greater.  The  F2  generation  should 
contain  1  white  seed  in  64.  It  happens  that 
none  were  obtained  in  this  generation.  The 
next  generation  should  contain  in  a  total  of 
64  individuals,  the  sorts  actually  observed  as 


12 

DQ    3 


• 


*  a.'--9! 


43H 

45     H     2   S     ® 

C3  r  1-1  >-i  1> 
P£H  I  I  °3  "^ 
bC  "g  "g  °  ct 

tttl     f 

6  6  cj      o 


"  BLENDING  "    INHERITANCE 

well  as  a  sort  which  would  produce  only  white 
seed,  the  progeny  namely  of  the  expected 
white  seed  of  F2,  but  as  that  was  not  obtained, 
the  all-white  plant  of  F3  could  not  be  obtained 
either.  The  expected  proportions  of  the  sev- 
eral classes  in  F3  are  given  for  comparison  with 
those  actually  obtained.  The  agreement  be- 
tween expected  and  observed  is  so  good  as  to 
make  it  seem  highly  probable  that  Nilsson- 
Ehle's  explanation  is  correct.  Corroborative 
evidence  in  the  case  of  maize  has  been  obtained 
by  Dr.  E.  M.  East  (Am.  Naturalist,  Feb.,  1910). 
This  work  introduces  us  to  a  new  principle 
which  may  have  important  theoretical  conse- 
quences. If  a  character  ordinarily  represented 
by  a  single  unit  in  the  germ-plasm  may  become 
represented  by  two  or  more  such  units  identi- 
cal in  character,  then  we  may  expect  it  to  domi- 
nate more  persistently  in  crosses,  fewer  reces- 
sives  being  formed  in  F2  and  subsequent  gen- 
erations. Further,  if  duplication  of  a  unit  tends 
to  increase  its  intensity,  as  seems  probable, 
then  we  have  in  this  process  a  possible  expla- 
nation of  quantitative  variation  in  characters 
which  are  non-Mendelian,  or  at  any  rate  do  not 
10  133 


HEREDITY 

conform  with  a  simple  Mendelian  system.  Con- 
sider, for  example,  the  matter  of  size  and 
skeletal  proportions  in  rabbits.  It  is  perfectly 
clear  from  the  experiments  described  that  in 
such  cases  no  dominance  occurs,  and  also  that 
no  segregation  of  a  simple  Mendelian  character 
takes  place,  but  it  is  not  certain  that  the  ob- 
served facts  may  not  be  explained  by  the  com- 
bined action  of  several  similar  but  independent 
factors,  the  new  principle  which  Nilsson-Ehle 
has  brought  to  our  attention.  Let  us  apply 
such  a  hypothesis  to  the  case  in  hand. 

Suppose  a  cross  be  made  involving  ear- 
lengths  of  approximately  4  and  8  inches  respec- 
tively, as  in  one  of  the  crosses  made.  The  Ft 
young  are  found  to  have  ears  about  6  inches 
long,  the  mean  of  the  parental  conditions,  and 
the  F2  young  vary  about  the  same  mean  con- 
dition. If  a  single  Mendelian  unit-character 
made  the  difference  between  a  4  inch  and  an  8 
inch  ear,  the  F2  young  should  be  of  three 
classes  as  follows: 

Classes  4  in.  6  in.  8  in. 

Frequencies          121 
134 


"BLENDING"    INHERITANCE 

(Compare  Fig.  47,  bottom  left.)    The  grand- 
parental  conditions  should  in  this  case  reappear 


.1 


i. 


ih 


i 


i 


FIG.  47.  —  Diagrams  to  show  the  number  and  size  of  the  classes 
of  individuals  to  be  expected  from  a  cross  involving  Mende- 
lian  segregation  without  dominance.  One  Mendelian  unit 
involved,  bottom  left;  two  units,  middle  left;  three  units, 
top  left;  four  units,  right. 

in  half  the  young.     This  clearly  does  not  oc- 
cur in  the  rabbit  experiment.    But  if  two  unit- 

135 


HEREDITY 

characters  were  involved,  FA  would  be  un- 
changed, all  6  inches,  yet  the  F2  classes  would 
be  more  numerous,  viz.,  4,  5,  6,  7,  and  8  inches, 
and  their  relative  frequencies  as  shown  by  the 
height  of  the  columns  in  Fig.  47,  middle  left,  1,  4, 
6,  4, 1.  The  grandparental  states  would  now  re- 
appear in  %  of  the  F2  young,  while  %  would  be 
intermediate.  It  is  certain,  however,  that  in  rab- 
bits the  grandparental  conditions,  if  they  re- 
appear at  all,  do  not  reappear  with  any  such 
frequency  as  this. 

If  three  independent  size-factors  were  in- 
volved in  the  cross,  the  Fx  individuals  should  all 
fall  in  the  same  middle  group,  as  before,  viz. 
6  inches,  but  the  F2  classes  should  number 
seven?  and  their  relative  frequencies  would 
be  as  shown  in  Fig.  47,  top  left.  For  4  independ- 
ent size-factors,  the  F2  classes  would  be  more 
numerous  still,  viz.,  9  (Fig.  47,  right),  and  the 
extreme  ear-size  of  either  grandparent  would 
be  expected  to  reappear  in  only  one  out  of  256 
offspring,  while  considerably  more  than  half 
of  them  would  fall  within  the  closely  inter- 
mediate classes  included  between  5%  and  61/2 
inches,  the  three  middle  classes  of  the  diagram. 

136 


"BLENDING"   INHERITANCE 

With  six  size-characters,  the  extreme  size  of  a 
grandparent  would  reappear  no  oftener  than 
once  in  4000  times,  while  with  a  dozen  such 
independent  characters  it  would  recur  only 
once  in  some  17,000,000  times.  It  would  be 
remarkable  if  under  such  conditions  the  ex- 
treme size  were  ever  recovered  from  an  ordi- 
nary cross. 

There  is  one  means  by  which  we  can  deter- 
mine with  certainty  whether  in  a  particular 
case  of  seemingly  blending  inheritance  segre- 
gation does  or  does  not  occur,  namely,  by  com- 
paring the  variability  of  the  Fx  and  the  F2 
generations.  If  segregation  does  not  occur,  F2 
should  be  no  more  variable  than  F1?  whereas 
if  segregation  does  occur,  F2  should  be  more 
variable.  For,  in  a  segregating  system,  the  F± 
individuals  should  all  fall  in  a  middle,  inter- 
mediate group,  but  the  F2  individuals  should 
be  distributed  also  in  classes  more  remote  from 
a  strictly  intermediate  position,  that  is,  they 
should  be  more  variable.  But,  in  a  non-segre- 
gating system,  Fj  and  F2  individuals  alike 
should  fall  in  the  same  intermediate  group, 
that  is,  they  should  have  the  same  variability. 

137 


HEKEDITY 

The  matter  should  be  easy  of  determination 
by  observation  of  considerable  numbers  of  F! 
and  F2  offspring.  Investigations  are  now  in 
progress  to  test  this  matter. 

My  colleague,  Dr.  East,  has  found  clear  evi- 
dence that,  in  maize,  size-characters,  although 
they  give  a  blending  result  in  F±,  nevertheless 
give  segregation  in  F2.  The  character  to  be  con- 
sidered relates  to  length  of  ear  in  corn.  A  single 
illustration  will  suffice.  The  variation  in  two 
pure  varieties  is  shown  in  the  two  upper  rows  of 
Fig.  48.  The  "  Length  "  of  each  class  is  given 
in  centimetres,  its  frequency  just  below  at  "  No. 
Var., ' '  abbreviation  for  number  of  variates.  The 
variation  in  the  F±  offspring  obtained  by  cross- 
ing the  two  pure  varieties  is  shown  in  the  third 
row,  and  that  of  the  F2  offspring  in  the  lowest 
row.  Note  that  the  variability  in  the  F±  gen- 
eration is  not  increased;  its  range  is  interme- 
diate between  the  range  in  the  parental  varie- 
ties. In  the  F2  generation,  however,  the  varia- 
bility is  so  increased  that  it  includes  almost 
the  entire  range  of  both  parental  varieties,  to- 
gether with  the  intervening  region. 

In  the  light  of  this  evidence  it  is  clear  that 
138 


FIG.  48.  —  Photographs  to  show  variation  in  ear  length  of  two  varie- 
ties of  maize  (upper  row),  of  their  Fl  offspring  (second  row),  and  of 
their  F2  offspring  (third  row).  (After  East.) 


"  BLENDING  »   INHEKITANCE 

in  maize,  seemingly  blending  is  really  segre- 
gating inheritance,  but  with  entire  absence  of 
dominance,  and  it  seems  probable  that  the  same 
will  be  found  to  be  true  among  rabbits  and 
other  mammals;  failure  to  observe  it  hitherto 
is  probably  due  to  the  fact  that  the  factors 
concerned  are  numerous.  For  the  greater  the 
number  of  factors  concerned,  the  more  nearly 
will  the  result  obtained  approximate  a  com- 
plete and  permanent  blend.  As  the  number 
of  factors  approaches  infinity,  the  result  will 
become  identical  with  a  permanent  blend. 

Theoretically  it  is  important  to  know  whether 
segregating  units  are  involved  in  inheritance 
which  we  call  blending;  practically  it  does  not 
matter  much,  since  if  these  units  are  only  as 
numerous  as  six  or  eight  it  will  be  practically 
impossible  to  undo  the  effects  of  a  cross  and  to 
recover  again  the  conditions  obtaining  previous 
to  the  cross.  The  great  majority  of  the  offspring 
both  in  the  first  and  in  subsequent  generations 
following  the  cross  will  be  strictly  intermediate 
between  the  conditions  crossed  whether  several 
units,  an  infinite  number  of  units,  or  no  units 
at  all  are  involved. 

139 


HEREDITY 

A  practical  question  of  some  importance  is 
how  to  manipulate  simultaneously  blending  (or 
seemingly  blending)  and  Mendelian  inheritance. 
This  must  be  by  a  system  of  line-breeding  in 
alternate  generations,  not  in  successive  genera- 
tions. To  test  the  practicability  of  this  matter 
I  several  years  ago  set  myself  the  task  of  com- 
bining in  one  race  the  large  size  of  some  lop- 
eared,  yellow  rabbits  which  I  had,  with  the 
albino  character  of  some  small  white  rabbits 
of  common  race.  A  first  cross  produced  gray 
rabbits  of  intermediate  size,  but  no  white  ones. 
On  inbreeding  the  gray  animals,  there  were 
obtained  in  F2  white  young  of  intermediate 
size.  These  were  now  crossed  again  with  the 
original  yellow  stock,  and  again  colored  young 
were  obtained,  but  now  with  %  of  the  desired 
increase  in  size.  These  bred  inter  se  again 
produced  albinos,  this  time  of  the  %  size.  A 
third  cross  with  the  original  large  stock  brought 
the  size  up  to  %  of  that  desired,  and  combined 
it  in  F2  with  the  desired  albinism.  Having 
satisfied  myself  of  the  correctness  of  the 
method,  the  experiment  was  now  discontinued. 
By  further  crosses,  especially  with  a  fresh  lop- 

140 


"  BLENDING  »   1NHEEITANCE 

eared  stock,  to  avoid  ill-effects  of  inbreeding, 
the  size  could  have  been  still  further  increased, 
with  judicious  selection  doubtless  up  to  the  ex- 
treme size  of  colored  lop-eared  rabbits. 

The  general  conclusion  to  be  drawn  is  that 
in  attempting  to  combine  in  one  race  by  cross- 
breeding characters  which  exist  separately  in 
different  races,  one  should  first  inquire  very 
carefully  how  each  character,  in  which  the  races 
differ,  behaves  in  transmission,  for  on  the  an- 
swer to  this  question  should  depend  the  mode 
of  procedure  to  be  chosen. 

If  simple  Mendelian  characters  only  are  con- 
cerned, nothing  is  required  but  to  cross  the  two 
races  and  select  from  the  second  generation 
offspring  the  desired  combination.  If  blending 
characters  only  are  concerned  and  Fx  yields 
the  desired  blend,  this  is  secure  without  fur- 
ther procedure,  except  possibly  selection  to  re- 
duce its  variability;  but  if  the  desired  blend 
is  not  yet  secured,  further  back-crossing  with 
one  race  or  the  other  may  be  necessary.  If, 
finally,  both  blending  and  Mendelian  characters 
are  simultaneously  involved  in  a  cross,  then  the 
method  of  combined  line-breeding  and  selec- 

141 


HEEEDITY 

tion  in  alternate  generations,  already  described, 
should  be  adopted. 


BIBLIOGRAPHY 

CASTLE,  W.  E. 

1909.  (See  Bibliography  to  Chapter  V.) 
EAST,  E.  M. 

1910.  "A  Mendelian  Interpretation  of  Variation  that  is 
Apparently    Continuous."      American    Naturalist,    44, 
pp.  65-82. 

EMERSON,  R.  A. 

1910.     "The  Inheritance  of  Sizes  and  Shapes  in  Plants." 

American  Naturalist,  44,  pp.  739-746. 
NILSSON-EHLE,  H. 

1909.    "Kreuzungsuntersuchungen  an  Hafer  und  Weizen." 
Lunds  Universitets  Arsskrift,  5,  No.  2,  122  pp. 


CHAPTER   IX 

THE   EFFECTS    OF   INBEEEDING 

WHAT  is  the  probable  source  of  the 
evil  effects  which  have  been  fre- 
quently   observed    to    follow    in- 
breeding? 

By-  inbreeding  we  mean  the  mating  of  closely 
related  individuals.  As  there  are  different  de- 
grees of  relationship  between  individuals,  so 
there  are  different  degrees  of  inbreeding.  The 
closest  possible  inbreeding  occurs  among  plants 
in  what  we  call  self-pollination,  in  which  the 
egg-cells  of  the  plant  are  fertilized  by  pollen- 
cells  produced  by  the  same  individual.  A  simi- 
lar phenomenon  occurs  among  some  of  the 
lower  animals,  notably  among  parasites.  But 
in  all  the  higher  animals,  including  the  domes- 
ticated ones,  such  a  thing  is  impossible  because 
of  the  separateness  of  the  sexes.  For  here  no 
individual  produces  both  eggs  and  sperm.  The 

143 


HEREDITY 

nearest  possible  approach  to  self-pollination  is 
in  such  cases  the  mating  of  brother  with  sister, 
or  of  parent  with  child.  But  this  is  less  close 
inbreeding  than  occurs  in  self-pollination,  for 
the  individuals  mated  are  not  in  this  case  iden- 
tical zygotes,  though  they  may  be  similar  ones. 

It  has  long  been  known  that  in  many  plants 
self-pollination  is  habitual  and  is  attended  by 
no  recognizable  ill-effects.  This  fortunate  cir- 
cumstance allowed  Mendel  to  make  his  remark- 
able discovery  by  studies  of  garden-peas,  in 
which  the  flower  is  regularly  self -fertilized,  and 
never  opens  at  all  unless  made  to  do  so  by 
some  outside  agency.  Self-pollination  is  also 
the  rule  in  wheat,  oats,  and  the  majority  of  the 
other  cereal  crops,  the  most  important  econom- 
ically of  cultivated  plants.  Crossing  can  in  such 
plants  be  brought  about  only  by  a  difficult 
technical  process,  so  habitual  is  self-pollina- 
tion. And  crossing,  too,  in  such  plants  is  of 
no  particular  benefit,  unless  by  it  one  desires 
to  secure  new  combinations  of  unit-characters. 

In  maize,  or  Indian  corn,  however,  among 
the  cereals,  the  case  is  quite  different.  Here 
enforced  self-pollination  results  in  small  un- 

144 


EFFECTS    OF   INBREEDING 

productive  plants,  lacking  in  vigor.  But  racial 
vigor  is  fully  restored  by  a  cross  between  two 
depauperate  unproductive  individuals  obtained 
by  self-fertilization,  as  has  recently  been  shown 
by  Shull.  This  result  is  entirely  in  harmony 
with  those  obtained  by  Darwin,  who  showed 
by  long-continued  and  elaborate  experiments 
that  while  some  plants  do  not  habitually  cross 
and  are  not  even  benefited  by  crossing,  yet  in 
many  other  plants  crossing  results  in  more 
vigorous  and  more  productive  offspring;  that 
further,  the  advantage  of  crossing  in  such  cases 
has  resulted  in  the  evolution  in  many  plants 
of  floral  structures,  which  insure  crossing 
through  the  agency  of  insects  or  of  the  wind. 

In  animals  the  facts  as  regards  close  fer- 
tilization are  similar  to  those-  just  described 
for  plants.  Some  animals  seem  to  be  indiffer- 
ent to  close  breeding,  others  will  not  tolerate 
it.  Some  hermaphroditic  animals  (those  which 
produce  both  eggs  and  sperm)  are  regularly 
self-fertilized.  Such  is  the  case,  for  example, 
with  many  parasitic  flat-worms.  In  other  cases 
self-fertilization  is  disadvantageous.  One  such 
case  I  was  able  to  point  out  some  fifteen  years 

145 


HEREDITY 

ago,  in  the  case  of  a  sea-squirt  or  tunicate, 
Ciona.  The  same  individual  of  Ciona  produces 
and  discharges  simultaneously  both  eggs  and 
sperm,  yet  the  eggs  are  rarely  self-fertilized, 
for  if  self-fertilization  is  enforced  by  isolation 
of  an  individual,  or  if  self-fertilization  is 
brought  about  artificially  by  removing  the  eggs 
and  sperm  from  the  body  of  the  parent  and 
mixing  them  in  sea-water,  very  few  of  the 
eggs  develop,  —  less  than  10%.  But  if  the 
eggs  of  one  individual  be  mingled  with  the 
sperm  of  any  other  individual  whatever,  prac- 
tically all  of  the  eggs  are  fertilized  and 
develop. 

In  the  great  majority  of  animals,  as  in  many 
plants,  self-fertilization  is  rendered  wholly  im- 
possible by  separation  of  the  sexes.  The  same 
individual  does  not  produce  both  eggs  and 
sperm,  but  only  one  sort  of  sexual  product. 
But  among  sexually  separate  animals  the  same 
degree  of  inbreeding  varies  in  its  effects.  The 
closest  degree,  mating  of  brother  with  sister, 
has  in  some  cases  no  observable  ill-effects. 
Thus,  in  the  case  of  a  smal]  fly,  Drosophila, 
my  pupils  and  I  bred  brother  with  sister  for 

146 


EFFECTS    OF   INBREEDING 

fifty-nine  generations  in  succession  without  ob- 
taining a  diminution  in  either  the  vigor  or  the 
fecundity  of  the  race,  which  could  with  cer- 
tainty be  attributed  to  that  cause.  A  slight 
diminution  was  observed  in  some  cases,  but 
this  was  wholly  obviated  when  parents  were 
chosen  from  the  more  vigorous  broods  in  each 
generation.  Nevertheless  crossing  of  two  in- 
bred strains  of  Drosophila,  both  of  which  were 
doing  well  under  inbreeding,  produced  off- 
spring superior  in  productiveness  to  either 
inbred  strain.  Even  in  this  case,  therefore, 
though  inbreeding  is  tolerated,  cross-breeding 
has  advantages. 

In  the  case  of  many  domesticated  animals, 
it  is  the  opinion  of  experienced  breeders,  sup- 
ported by  such  scientific  observations  as  we 
possess,  that  decidedly  bad  effects  follow  con- 
tinuous inbreeding.  Bos  ( '94)  practiced  con- 
tinuous inbreeding  with  a  family  of  rats  for 
six  years.  No  ill-effects  were  observed  during 
the  first  half  of  the  experiment,  but  after  that 
a  rapid  decline  occurred  in  the  vigor  and  fer- 
tility of  the  race.  The  average-sized  litter  in 
the  first  half  of  the  experiment  was  about  7.5, 

147 


HEREDITY 

but  in  the  last  year  of  the  experiment  it  had 
fallen  to  3.2,  and  many  pairs  were  found  to 
be  completely  sterile.  Diminution  in  size  also 
attended  the  inbreeding,  at  the  end  amounting 
in  the  case  of  males  to  between  8  and  20  %. 

Experiments  made  by  Weismann  confirm 
those  of  Bos  as  regards  the  falling  off  in  fer- 
tility due  to  inbreeding.  For  eight  years  Weis- 
mann bred  a  colony  of  mice  started  from  nine 
individuals,  —  six  females  and  three  males. 
The  experiment  covered  29  generations.  In 
the  first  10  generations  the  average  number  of 
young  to  a  litter  was  6.1;  in  the  next  10  gen- 
erations, it  was  5.6;  and  in  the  last  9  genera- 
tions, it  had  fallen  to  4.2.  But  sweeping 
generalizations  cannot  be  drawn  from  these 
cases.  Each  species  of  animal  must  probably 
be  tested  for  itself  before  we  shall  know  what 
the  exact  effects  of  inbreeding  are  in  that  case. 
In  guinea-pigs,  a  polydactylous  race  built  up 
by  the  closest  inbreeding  out  of  individuals  all 
descended  from  one  and  the  same  individual 
has  now  been  in  existence  for  ten  years.  It 
consists  of  one  of  the  largest  and  most  vigor- 
ous strains  of  guinea-pigs  that  I  have  ever 

148 


EFFECTS    OF   INBKEEDING 

seen,  and  has  shown  no  indications  of  dimin- 
ished fertility. 

In  the  production  of  pure  breeds  of  sheep, 
cattle,  hogs,  and  horses  inbreeding  'has  fre- 
quently been  practiced  extensively,  and  where 
in  such  cases  selection  has  been  made  of  the 
more  vigorous  offspring  as  parents,  it  is  doubt- 
ful whether  any  diminution  in  size,  vigor,  or 
fertility  has  resulted.  Nevertheless  it  very 
frequently  happens  that  when  two  pure  breeds 
are  crossed,  th6  offspring  surpass  either  pure 
race  in  size  and  vigor.  This  is  the  reason  for 
much  cross-breeding  in  economic  practice,  the 
object  of  which  is  not  the  production  of  a  new 
breed,  but  the  production  for  the  market  of 
an  animal  maturing  quickly  or  of  superior  size 
and  vigor.  The  inbreeding  practiced  in  form- 
ing a  pure  breed  has  not  of  necessity  dimin- 
ished vigor,  but  a  cross  does  temporarily  (that 
is  in  the  Fx  generation)  increase  vigor  above 
the  normal.  Now  why  should  inbreeding  un- 
attended by  selection  decrease  vigor,  and  cross- 
breeding increase  it?  We  know  that  inbreed- 
ing tends  to  the  production  of  homozygous 
conditions,  whereas  cross-breeding  tends  to 
11  149 


HEEEDITY 

produce  heterozygous  conditions.  Under  self- 
pollination  for  1  generation  following  a  cross, 
half  the  offspring  become  homozygous;  after 
2  generations,  %  of  the  offspring  are  homo- 
zygous ;  after  3  generations  %  are  homozygous, 
and  so  on.  So  if  the  closest  inbreeding  is 
practiced  there  is  a  speedy  return  to  homo- 
zygous, pure  racial  conditions.  We  know  fur- 
ther that  in  some  cases  at  least  heterozygotes 
are  more  vigorous  than  homozygotes.  The 
heterozygous  yellow  mouse  is  a  vigorous  lively 
animal;  the  homozygous  yellow  mouse  is  so 
feeble  that  it  perishes  as  soon  as  produced, 
never  attaining  maturity.  Cross-breeding  has, 
then,  the  same  advantage  over  close-breeding 
that  fertilization  has  over  parthenogenesis.  It 
brings  together  differentiated  gametes,  which, 
reacting  on  each  other,  produce  greater  meta- 
bolic activity. 

Inbreeding,  also,  by  its  tendency  to  secure 
homozygous  combinations,  tends  to  bring  to 
the  surface  latent  or  hidden  recessive  charac- 
ters. If  these  are  in  nature  defects  or  weak- 
nesses of  the  organism,  such  as  albinism  and 
feeble-mindedness  in  man,  then  inbreeding  is 

150 


EFFECTS    OF   INBREEDING 

distinctly  bad.  Existing  legislation  against 
the  marriage  of  near-of-kin  is,  therefore,  on 
the  whole,  biologically  justified.  On  the  other 
hand,  continual  crossing  only  tends  to  hide 
inherent  defects,  not  to  exterminate  them;  and 
inbreeding  only  tends  to  bring  them  to  the 
surface,  not  to  create  them.  We  may  not, 
therefore,  lightly  ascribe  to  inbreeding  or  in- 
termarriage the  creation  of  bad  racial  traits, 
but  only  their  manifestation.  Further,  any 
racial  stock  which  maintains  a  high  standard 
of  excellence  under  inbreeding  is  certainly  one 
of  great  vigor,  and  free  from  inherent  defects. 
The  animal  breeder  is  therefore  amply  jus- 
tified in  doing  what  human  society  at  present 
is  probably  not  warranted  in  doing,  —  viz.  in 
practicing  close  inbreeding  in  building  up 
families  of  superior  excellence  and  then  keep- 
ing these  pure,  while  using  them  in  crosses 
with  other  stocks.  For  an  animal  of  such  a 
superior  race  should  have  only  vigorous,  strong 
offspring  if  mated  with  a  healthy  individual 
of  any  family  whatever,  within  the  same  spe- 
cies. For  this  reason  the  production  of 
"  thoroughbred  "  animals  and  their  use  in 

151 


HEREDITY 

crosses  is  both  scientifically  correct  and  com- 
mercially remunerative. 

BIBLIOGRAPHY 

BOS,    RlTZEMA. 

1894.     "  Untersuchungen  ueber  die  Folgen  der  Zucht  in 
engster  Blutverwandtschaft."    Biol.  CentrbL,  14,  pp.  75- 
81. 
CASTLE,  W.  E.,  CARPENTER,  F.  W.,  CLARK,  A.  H.,  MAST, 

S.  0.,  and  BARROWS,  W.  M. 

"The  Effects  of  Inbreeding,  Cross-breeding  and  Selec- 
tion upon  the  Fertility  and  Variability  of   Drosophila." 
Proc.  Amer.  Acad.  Arts  and  Sci.,  41,  pp.  731-786. 
GUAITA,  G.  VON. 

1898.  "Versuche  mit  Kreuzungen  von  verschiedenen 
Rassen  des  Hausmaus."  Ber.  naturf.  Gesellsch.  zu 
Freiburg,  10,  pp.  317-332.  [Contains  observations  of 
Weismann.] 


CHAPTER   X 

HEREDITY   AND   SEX 

THE  value  of  a  domesticated  animal  often 
depends  in  considerable  measure  on  its 
sex.  Therefore,  if  a  means  could  be 
devised  for  controlling  the  sex  of  offspring,  it 
would  be  of  great  economic  value  to  the  breeder. 
Endless  attempts  have  been  made  to  do  this, 
and  occasionally  a  claim  of  success  has  been 
made,  but  none  of  these  claims  has  withstood 
the  test  of  critical  analysis  or  experiment.  The 
hypotheses  advanced  to  explain  how  sex  may 
be  controlled  have  been  of  the  most  varied 
character.  In  some  the  determination  of  sex 
has  been  supposed  to  inhere  in  the  nature  of 
the  parents,  in  others  it  is  referred  to  condi- 
tions of  the  gametes  themselves. 

Relative  age  or  vigor  of  the  parents  have 
been  supposed  to  influence  sex  in  various  ways. 
The  same  idea  has  been  advanced  regarding 

153 


HEREDITY 

the  gametes  themselves,  it  being  supposed  that 
early  or  late  fertilization  of  the  egg  might 
influence  its  sex.  Experimental  evidence,  how- 
ever, as  to  these  several  hypotheses  is  wholly 
negative,  when  one  eliminates  other  possible 
factors  from  the  experiment.  Everything 
points  to  the  conclusion  that  sex  rests  in  the 
last  analysis  upon  gametic  differentiation,  just 
as  the  color  of  a  guinea-pig  in  a  mixed  race 
of  blacks  and  whites  depends  upon  whether  the 
gametes  which  unite  to  produce  it  carry  black 
or  white.  As  the  heterozygous  black  guinea- 
pig  forms  black-producing  and  white-producing 
gametes  in  equal  numbers,  so  there  is  reason 
to  think  male-producing  and  female-producing 
gametes  are  formed  in  equal  numbers  by  the 
parent,  in  many  cases  at  least.  But  is  it  not 
possible  that  there  may  exist  individuals  which 
produce  the  two  sorts  of  gametes  in  unequal 
numbers,  and  so  would  have  a  tendency  to 
produce  more  offspring  of  one  sex  than  of  the 
other?  Perhaps  so,  though  we  have  no  evi- 
dence that  such  a  condition,  if  it  does  exist,  is 
transmitted  from  one  generation  to  another. 
On  this  point  I  made  experimental  observa- 

154 


HEREDITY   AND   SEX 

tions  upon  guinea-pigs  extending  over  a  series 
of  years.  Oftentimes  I  found  an  individual 
that  produced  more  offspring  of  one  sex  than 
of  the  other,  but  this  was  probably  due  merely 
to  chance  deviations  from  equality.  I  could 
get  no  evidence  that  the  condition  was  inher- 
ited, though  the  experiment  was  continued 
through  as  many  as  seven  generations,  includ- 
ing several  hundred  offspring. 

The  essential  difference  between  a  female 
and  a  male  individual  is  that  one  produces 
eggs,  the  other  sperm.  All  other  differences 
are  secondary  and  dependent  largely  upon  the 
differences  mentioned.  If  in  the  higher  ani- 
mals (birds  and  mammals)  the  sex-glands  (i.  e. 
the  egg-producing  and  sperm-producing  tis- 
sues) are  removed  from  the  body,  the  super- 
ficial differences  between  the  sexes  largely  dis- 
appear. In  insects,  however,  the  secondary 
sex-characters  seem  to  be  for  the  most  part 
uninfluenced  by  presence  or  absence  of  the 
sex-glands.  Their  differentiation  occurs  in- 
dependently though  simultaneously  with  that 
of  the  sex-glands. 

The  egg  or  larger  gamete  (the  so-called 
155 


HEREDITY 

macro-gamete)  in  all  animals  is  non-motile  and 
contains  a  relatively  large  amount  of  reserve- 
food  material  for  the  maintenance  of  the  de- 
veloping embryo.  This  reserve-food  material 
it  is  the  function  of  the  mother  to  supply.  In 
the  case  of  some  animals,  for  example  flat- 
worms  and  mollusks,  the  food-supply  of  the 
embryo  is  not  stored  in  the  egg-cell  itself,  but 
in  other  cells  associated  with  it,  and  which 
break  down  and  supply  nourishment  to  the 
developing  embryo  derived  from  the  fertilized 
egg.  Again,  as  in  the  mammals,  the  embryo 
may  derive  its  nourishment  largely  from  the 
maternal  tissues,  the  embryo  remaining  like  a 
parasite  within  the  maternal  body  during  its 
growth,  feeding  by  absorption.  But  in  all  cases 
alike  the  mother  supplies  the  larger  gamete 
and  the  food-material  necessary  to  carry  the 
zygote  through  its  embryonic  stages.  The 
father,  on  the  other  hand,  furnishes  the  bare 
hereditary  equipment  of  a  gamete,  with  the 
motor  apparatus  necessary  to  bring  it  into 
contact  with  the  egg-cell,  but  without  food  for 
the  developing  embryo  produced  by  fertiliza- 
tion. The  gamete  furnished  by  the  father  is 

156 


HEEEDITY   AND   SEX 

therefore    the    smaller    gamete,    the    so-called 
micro-gamete. 

From  the  standpoint  of  metabolism,  the 
female  is  the  more  advanced  condition;  the 
female  performs  the  larger  function,  doing  all 
that  the  male  does  in  furnishing  the  material 
basis  of  heredity  (a  gamete),  and  in  addition 
supplying  food  for  the  embryo.  As  regards 
the  reproductive  function,  the  female  is  the 
equivalent  of  the  male  organism,  plus  an  ad- 
ditional function,  —  that  of  supplying  the  em- 
bryo with  food.  When  we  come  to  consider  the 
structural  basis  of  sex,  we  find  reasons  for 
thinking  that  here,  too,  the  female  individual 
is  the  equivalent  of  the  male  plus  an  addi- 
tional element.  The  conclusion  has  very  natu- 
rally been  drawn  that  if  a  means  could  be 
devised  for  increasing  the  nourishment  of  the 
egg  or  embryo,  its  development  into  a  female 
should  be  thereby  insured,  while  the  reverse 
treatment  should  lead  to  the  production  of  a 
male.  But  in  practice  this  a  priori  expecta- 
tion is  not  fulfilled.  Better  nourishment  of  the 
mother  may  lead  to  the  production  of  more 
eggs,  but  not  of  more  female  offspring,  as  has 

157 


HEREDITY 

been  repeatedly  demonstrated  by  experiment. 
Also  poor  nutrition  of  the  mother  may  diminish 
the  number  of  eggs  which  she  liberates,  but  will 
not  increase  the  proportion  of  males  among  the 
offspring  produced. 

An  excellent  summary  of  evidence  on  this 
point  was  made  by  Cuenot  in  1900.  Attempts 
to  influence  the  sex  of  an  embryo  or  larva  by 
altered  nutrition  of  the  embryo  or  larva  itself 
have  proved  equally  futile.  Practically  the 
only  experimental  evidence  of  value  in  favor 
of  this  idea  has  been  derived  from  the  study 
of  insects,  and  this  is  capable  of  explanation 
on  quite  different  grounds  from  those  which 
first  suggest  themselves.  It  has  sometimes  been 
observed,  as  by  Mary  Treat  for  example,  that 
a  lot  of  insects  poorly  fed  produce  an  excess 
of  males.  In  such  lots,  however,  the  mortality 
is  commonly  high,  and  more  females  die  than 
males,  because  the  female  is  usually  larger  and 
requires  more  food  to  complete  its  development. 
The  fallacy  in  concluding  from  such  evidence 
that  scanty  nutrition  causes  individuals  which 
would  otherwise  become  females  to  develop 
into  males  was  indicated  years  ago  by  Riley. 

158 


HEREDITY   AND   SEX 

Nevertheless  an  argument  for  the  artificial  con- 
trol of  sex  based  on  such  evidence  is  from  time 
to  time  brought  forward,  as,  for  example,  a 
few  years  since  by  Schenk.  The  latest  advocate 
of  sex-control  by  artificial  means  is  an  Italian, 
Eusso  (1909).  He  claims  in  the  case  of  rabbits 
that  by  feeding  the  mother  on  lecithin  or  by 
injections  of  lecithin,  the  proportion  of  female 
births  may  be  increased.  His  evidence  in  sup- 
port of  this  claim  is,  however,  wholly  inade- 
quate, and  two  independent  repetitions  of  his 
experiments,  made  by  Basile  in  Italy  and  by 
Punnett  in  England,  have  given  entirely  nega- 
tive results. 

An  alternative  hypothesis  concerning  the  de- 
termination of  sex  has  been  steadily  gaining 
ground  during  the  last  ten  years,  that  sex  has 
its  beginning  in  gametic  differentiation  and  is 
finally  determined  beyond  recall  in  the  ferti- 
lized egg  by  the  nature  of  the  uniting  gametes. 
Instructive  in  this  connection  is  a  study  of 
parthenogenesis,  —  reproduction  by  unfertilized 
eggs.  But  before  entering  upon  this,  it  may 
be  well  to  review  briefly  the  changes  which 
regularly  take  place  in  the  egg  which  is  to  be 

159 


HEKEDITY 

fertilized,  and  compare  with  this  the  changes 
which  occur  in  eggs  not  to  Be  fertilized. 

In  each  cell  of  the  ordinary  animal  there 
occurs  a  characteristic  number  of  bodies  called 
chromosomes.  We  do  not  know  that  they  are 
any  more  important  than  other  cell  constitu- 
ents, but  we  know  their  history  better.  These 
are  contained  in  the  nucleus  of  the  cell,  and  at 
the  time  of  nuclear  diyision  they  are  found  at 
the  equator  of  the  division  spindle.  For  ex- 
ample, in  the  egg  of  the  mouse  (Fig.  4,  A),  the 
nucleus  is  seen  to  be  in  the  spindle  stage,  and 
its  chromosomes  are  gathered  together  at  the 
equator  of  the  spindle.  There  each  of  them 
regularly  splits  in  two,  and  one  derivative  goes 
to  either  end  of  the  spindle,  and  so  into  one  of 
the  daughter-nuclei.  Thus  each  new  nucleus 
has,  as  a  rule,  the  same  chromosome  composi- 
tion as  the  nucleus  from  which  it  was  derived. 

But  the  egg  which  is  to  be  fertilized  under- 
goes two  nuclear  divisions  in  succession,  in  only 
one  of  which  do  the  chromosomes  split  (see 
Fig.  4,  A-D).  In  the  other  division  the  chromo- 
somes separate  into  two  groups  without  split- 
ting, and  each  group  goes  into  a  different  cell 

160 


HEREDITY   AND   SEX 

product.  Consequently,  in  each  of  these  prod- 
ucts the  number  of  chromosomes  is  reduced  to 
half  what  it  is  in  the  cells  of  the  parental  body. 
Thus  in  the  egg  of  the  mouse,  by  maturation, 
the  number  of  chromosomes  becomes  reduced 
from  about  twenty-four  to  about  twelve. 

Similar  changes  occur  in  the  developing 
sperm-cell  (see  Fig.  5).  Starting  with  the 
double  or  2  N  chromosome  number,  there  are 
formed  by  two  nuclear  divisions,  with  only  one 
splitting  of  chromosomes,  four  cells,  each  with 
the  reduced  or  simplex  number  of  chromosomes, 
N.  Consequently,  when  the  sperm  enters  the 
egg  at  fertilization  it  brings  in  a  group  of  N 
chromosomes  (in  the  mouse  apparently  twelve), 
which,  added  to  the  egg-contribution  of  N 
chromosomes,  brings  the  number  in  the  new 
organism  again  up  to  2  N  (in  the  mouse  twenty- 
four). 

Now,  as  regards  the  maturation  of  partheno- 
genetic  eggs,  those  which  are  to  develop  with- 
out having  been  fertilized,  three  categories  of 
cases  deserve  separate  discussion.  The  simplest 
of  these  in  many  respects  is  found  among  the 
social  hymenoptera  (ants,  bees,  and  wasps). 

161 


HEEEDITY 

See  Fig.  49,  left  column.    The  eggs  are,  so  far 
as  we  can  discover,  all  of  a  single  type.    They 

BEE          XOTIFER       APHID 


AVORABLE 

OA/0/r/OA/S 


\  ^~7^^ 

\y.  \  . 


FIG.  49.  —  Diagram  of  sex-determination  in  parthenogenesis. 
First  row,  nuclear  condition  of  the  parthenogenetic  mother; 
second  row,  of  her  eggs  when  they  develop  without  reduction, 
after  forming  a  single  polar  cell;  third  row,  condition  of  the 
eggs  after  complete  maturation  —  the  fertilized  egg  in  each 
case  produces  a  male;  fourth  row,  nuclear  condition  of  the 
fertilized  egg,  always  a  female. 

undergo  maturation  in  the  manner  already  de- 
scribed, the  chromosomes  being  reduced  to  the 

162 


HEREDITY   AND   SEX 

N  or  simplex  number.  The  eggs  of  most  ani- 
mals, after  they  have  undergone  reduction,  are 
incapable  of  development  unless  fertilized,  but 
those  of  the  hymenoptera  may  develop  either 
fertilized  or  unfertilized.  In  the  former  case 
a  female  is  produced,  in  the  latter  a  male.  The 
simplex,  or  N  condition  is  in  this  case  the  male, 
the  duplex  or  2  N  condition  is  the  female,  natu- 
rally the  one  of  higher  metabolic  activity,  the 
one  which  forms  the  macro-gametes. 

In  an  earlier  chapter  I  explained  how  the 
development  of  the  sperm-cells  in  a  male  having 
the  reduced  or  simplex  number  of  chromosomes 
differs  from  that  in  the  ordinary  male.  Eefer- 
ence  to  Fig.  8  may  help  to  recall  this.  The 
cells  of  the  male  are  in  this  case  already  in  the 
reduced  or  simplex  condition,  N.  In  the  pro- 
duction of  the  sperms  the  reducing  division  is 
omitted  so  far  as  nuclear  components  are  con- 
cerned, so  that  each  sperm  formed  contains  the 
full  simplex  chromosome  number,  N.  If  it  were 
less,  the  gamete  formed  would  perhaps  not  be 
capable  of  transmitting  all  the  hereditary  char- 
acteristics of  an  individual. 

A  second  category  of  cases  (Fig.  49,  middle 
163 


HEREDITY 

column)  is  represented  by  such  simple  aquatic 
organisms  as  rotifers  and  small  Crustacea,  like 
Daphnia.  In  these  parthenogenesis  occurs  ex- 
clusively, when  the  food  supply  is  very  abun- 
dant and  conditions  otherwise  favorable,  whereas 
reproduction  by  fertilized  eggs  occurs  only 
when  external  conditions,  including  food-supply, 
are  not  good.  Under  favorable  conditions  only 
female  offspring  are  produced.  The  conclusion 
has  naturally  but  erroneously  been  drawn  that 
good  nutrition  in  itself  favors  the  production 
of  females  in  animals  generally,  which  is  not 
true.  The  egg  produced  by  Daphnia,  or  by  a 
rotifer,  under  optimum  conditions  does  not  un- 
dergo reduction  (see  Fig.  49,  second  row).  It 
remains  in  the  2  N  condition,  forming  but  a 
single  polar  cell.  It  is  therefore  unprepared 
for  fertilization,  and  in  fact  it  is  not  fertilized. 
Its  sex  is  like  that  of  the  animal  which  formed 
it,  female.  Under  unfavorable  conditions,  how- 
ever, the  eggs  of  the  rotifer  and  of  Daphnia 
do  not  begin  development  until  they  have  un- 
dergone maturation.  They  are  also  of  two 
sizes  (Fig.  49,  third  row),  —  small  eggs,  which 
develop  without  fertilization  and  which  form 

164 


HEREDITY   AND   SEX 

males,  and  large  eggs,  which  require  fer- 
tilization, and  which  form  females.  In  this 
category  of  cases,  as  in  that  of  the  hymenop- 
tera,  the  egg  which  develops  in  the  2  N  condi- 
tion, either  from  failure  of  reduction  to  occur 
in  maturation  or  from  fertilization  following 
reduction,  forms  a  female;  whereas  the  egg 
which  develops  in  the  N  condition  forms  a 
male. 

In  a  third  category  of  cases  there  is  a  quan- 
titative difference  in  chromatin  between  male 
and  female,  just  as  in  the  foregoing  cases,  but 
this  does  not  amount  to  a  whole  set  of  chromo- 
somes, N,  but  to  only  a  partial  set,  one  or  two 
chromosomes  (see  Fig.  49,  right  column).  This 
category  of  cases  occurs  in  plant-lice  (aphids 
and  phylloxerans) ;  evidence  of  its  existence 
rests  chiefly  on  recent  observations  made  by 
von  Baehr  and  Morgan.  Females  are  formed 
by  parthenogenesis  without  reduction,  occurring 
under  favorable  conditions,  just  as  in  the  case 
of  rotifers.  Females  are  also  formed  by  fer- 
tilization following  reduction  under  unfavor- 
able conditions,  just  as  in  rotifers.  In  both 
cases  the  female  is  2  N.  Males  arise  only  by 
12  165 


HEREDITY 

parthenogenesis  under  unfavorable  conditions, 
just  as  in  rotifers,  but  the  reduction  which 
occurs  before  development  begins  is  partial 
only.  A  whole  set,  N,  of  chromosomes  is  not 
eliminated  in  maturation,  but  only  1  or  2  chro- 
mosomes. Hence  the  male  condition  here  is 
2  N  —  1  or  —  2.  The  condition  of  the  gametes 
formed,  however,  is  N  in  both  sexes.  In 
spermatogenesis,  division  of  the  germ-cells 
takes  place  into  N  and  N  -  -  1  daughter  cells, 
but  the  latter  degenerate  (like  the  non-nucleated 
cells  of  the  bee  and  wasp),  and  only  the  former 
produce  spermatozoa.  Hence  in  fertilization 
only  2  N  zygotes  are  produced,  which  are  in- 
variably female. 

Summarizing  the  three  categories  described, 
we  may  say  that  in  all  known  cases  of  par- 
thenogenesis, the  female  is  in  the  duplex,  2  N 
condition,  the  male  in  the  simplex  (N)  or  par- 
tially duplex  condition  (2  N  --  1,  or  2  N  —  2). 
The  female  in  all  cases  has  the  greater  chro- 
matin  content. 

In  a  great  many  insects  and  other  arthro- 
pods, which  are  not  parthenogenetic,  it  is 
known  that,  although  the  male,  like  the  female, 

166 


HEREDITY   AND   SEX 

develops  only  from  a  fertilized  egg,  neverthe- 
less the  male  possesses  fewer  chromosomes  than 


FIG.  50.  —  Diagram  of    sex-determination  when  the  female  is 
homozygous,  the  male  heterozygous. 

the  female.  In  such  cases  the  female  forms, 
as  in  cases  of  parthenogenesis,  only  N  gametes, 
but  the  male  forms  gametes  of  two  sorts,  N  and 

167 


HEEEDITY 

N  —  1  or  N  —  2  (see  Fig.  50).  In  consequence 
zygotes  of  two  sorts  result,  —  those  which  are 
2  N,  female,  and  those  which  are  2  N  —  1  or 
2  N  —  2,  male.  Thus  in  the  squash-bug,  Anasa- 
tristis,  according  to  Wilson,  the  mature  egg 
contains  11  chromosomes,  the  spermatozoa 
either  10  or  11  chromosomes,  the  two  sorts 
being  equally  numerous. 

Egg  11  +  sperm  11  produces  a  zygote  22  (2N),  a  female; 
Egg  11+  "  10  "  "  "  21(2N-l),amale. 

N  in  this  species  —  11 ;  2  N  ==  22,  the  female ; 
2  N  —  1  =  21,  the  male.  Males  and  females  are 
therefore  approximately  equal  in  number,  as  in 
most  animals  where  the  two  sexes  are  not  sub- 
ject to  unequal  mortality.  In  the  Mendelian 
sense  the  female  is  in  such  cases  a  homozygote, 
the  male  a  heterozygote.  The  sex  of  an  indi- 
vidual in  such  cases  depends  upon  which  sort 
of  a  sperm  chances  to  enter  the  egg. 

But  the  experimental  evidence  indicates 
that  both  as  regards  sex  and  as  regards 
heritable  characters  correlated  with  sex,  these 
relations  may  in  some  cases  be  reversed,  the 
female  being  heterozygous,  the  male  homozy- 

168 


HEREDITY   AND   SEX 

gous.  In  such  cases  there  is  reason  to  think 
that  structurally  the  male  is  2  N  but  the  female 
2N  +.  That  is,  the  female  is  still  the  equiva- 
lent of  the  male  plus  some  additional  element 
and  function.  A  structural  basis  in  the  chromo- 
somes for  such  a  condition  has  been  described 
by  Baltzer  in  the  case  of  the  sea-urchin.  He 
found  the  regular  duplex  number  of  chromo- 
somes in  the  male;  but  in  the  female,  while 
the  number  was  the  same,  one  of  the  chromo- 
somes was  larger  than  its  mate,  having  an  extra 
or  odd  element  attached  to  it.  In  such  a  case 
the  gametes  formed  by  the  male  would  all  be 
N,  but  those  formed  by  the  female  would  be 
of  two  sorts  equally  numerous,  viz.  N  and  N  + 
(see  Fig.  51).  Egg  N  fertilized  by  sperm  N 
would  produce  a  zygote  2  N,  a  male ;  egg  N  + 
fertilized  by  sperm  N  would  produce  a  zygote 
2  N  +,  a  female.  Hence  here,  as  in  other  ani- 
mals, the  sexes  would  be  approximately  equal, 
but  the  sex  of  a  particular  individual  would 
depend  upon  which  sort  of  egg  gave  rise  to  it. 
Upon  the  existence,  as  in  the  foregoing  cases, 
of  an  unpaired  or  odd  structural  element  in 
the  egg,  may  perhaps  depend  the  explanation 

169 


HEREDITY 


of  a  curious  sort  of  heredity  known  as  sex- 
limited  heredity. 


FIG.  51.  —  Diagram  of  sex-determination  when  the  female  is 
heterozygous,  the  male  homozygous. 

Every  one  who  knows  anything  about  poultry 
is  acquainted  with  the  popular  American  breed 
known  as  the  barred  Plymouth  Eock.  In  this 

170 


HEREDITY   AND   SEX 

breed  the  feathers  are  marked  with  alternate 
bars  of  darker  and  lighter  black.  Pure  barred 
Eocks  breed  true,  but  when  crossed  with  other 
breeds,  the  male  proves  to  be  homozygous,  the 
female,  heterozygous  in  barring.  For  the  male 
Eock  crossed  with  a  non-barred  breed  produces 
only  barred  offspring  in  both  sexes,  but  the 
female  Eock  crossed  with  the  same  non-barred 
breed  produces  offspring  approximately  half 
of  which  are  barred,  the  other  half  being  non- 
barred.  Further,  the  barred  individuals  in  this 
cross  are  invariably  males,  the  non-barred  ones 
being  females.  Accordingly,  the  distribution  of 
barring  and  non-barring  in  the  cross  is  sex- 
limited. 

The  barred  offspring  produced  by  a  cross 
between  barred  Plymouth  Eocks  and  a  non- 
barred  breed,  whether  those  offspring  are  males 
or  females,  prove  to  be  heterozygous  in  barring, 
as  we  should  expect,  the  barring  factor  having 
been  received  only  from  one  parent,  the  barred 
one.  Further,  the  non-barred  offspring  pro- 
duced by  a  barred  Eock  female  crossed  with 
a  non-barred  breed,  do  not  transmit  barring, 
hence  they  are  pure  recessives  as  regards  bar- 

171 


HEREDITY 

ring.  Hence,  also,  we  are  forced  to  conclude, 
as  already  suggested,  the  female  of  the  pure 
barred  Rock  breed  is  heterozygous  as  re- 
gards barring,  and  transmits  the  character  only 
to  her  male  offspring,  her  female  offspring 
(if  the  father  is  non-barred)  neither  being 
themselves  barred  nor  being  able  to  transmit 
barring. 

A  pure  Plymouth  Eock  race  breeds  true  to 
barring  merely  because  all  its  males  are  pure, 
for  the  females  are  not  pure.  This  is  shown 
by  the  following  experiment.  If  a  heterozygous 
barred  male,  produced  by  a  cross  between  a 
Eock  and  a  non-barred  breed,  is  crossed  with 
barred  females,  either  those  of  a  pure  Eock 
race  or  those  produced  by  a  cross,  the  result 
is  the  same.  The  male  offspring  are  all  barred ; 
the  females,  half  of  them  barred,  half  non- 
barred.  This  result  shows  that  all  barred 
females  alike  are  heterozygous  in  barring. 

Sex-limited  inheritance  such  as  this  finds  at 
the  present  time  its  most  probable  explanation 
in  the  existence  in  the  egg  of  an  extra  or  plus 
element  never  found  in  the  sperm,  this  element 
pairing  with  the  sex-limited  character  in  the 

173 


HEEEDITY   AND   SEX 

reduction  division.  Thus,  in  the  barred  Rock, 
calling  barring  B,  the  male  of  pure  race  is 
plainly  B  B  and  every  sperm  is  B.  But  the 
female  clearly  contains  only  one  B  and  can- 


FEMALE 


MALE 


FIG.  52.  —  Diagram  of  sex-limited  inheritance  when  the  female 
is  a  heterozygote,  as  in  barred  fowls.  X,  female  sex  deter- 
miner; B,  barring. 

not  be  made  to  contain  two.  Perhaps  a  second 
B  is  kept  out  by  some  structural  element,  X, 
the  distinctive  structural  element  of  the  female 
individual.  Then  the  eggs  will  be  of  two 
sorts:  B  and  X.  Since  the  sperms  are  all  B, 
the  first  type  of  egg  when  fertilized  will  con- 
tain B  B,  a  homozygous  barred  individual  and 

173 


HEREDITY 

a  male,  since  it  lacks  X;  the  second  type  will 
contain  B  X,  a  bird  heterozygous  in  barring, 
and  a  female,  since  it  contains  X.  This  agrees 
with  the  experimental  result  (see  Fig.  52). 

A  heterozygous  barred  male  will  form  two 
kinds  of  sperm,  only  one  of  which  will  contain 
B,  If  such  a  male  be  mated  with  a  barred 
female,  four  sorts  of  zygotes  should  result,  as 
follows : 

Gametes  of  heterozygous  barred  male  =  B  and  — , 
Gametes  of  barred  female  =  B  and  X, 

Zygotes  =  BB  (homozygous  barred  male);  B  — 

(heterozygous  barred  male),  B'X  (barred  female),  and 

— X  (non-barred  female). 

The  observed  result  of  this  cross  accords 
fully  with  the  foregoing  expectation. 

The  sex-limited  inheritance  of  barring  in 
fowls  may  be  explained,  as  we  have  just  seen, 
on  the  assumption  that  the  female  is  the  hetero- 
zygous sex.  The  same  is  true  of  sex-limited 
inheritance  in  canary-birds  and  in  the  moth, 
Abraxas,  according  to  Bateson  and  Doncaster. 
But  these  relations  are  exactly  reversed  in  the 
pomace-fly,  Drosophila  ampelophila,  according 
to  Morgan. 

174 


HEEEDITY   AND    SEX 

In    Drosophila    the    female    is    apparently 

homozygous    as    regards    some    cell-structure, 

X,    which   in   the   male   is    never    represented 

more  than  once.     Accordingly  the  formula  of 

FEMALE  MALE 


FIG.  53.  —  Diagram  of  sex-limited  inheritance  when  the  female 
is  a  homozygote,  as  in  the  red-eyed  Drosophila.  X,  sex- 
determiner;  R,  red-eyes. 

the  female  is  in  such  cases  X  X ;  that  of  the 
male,  X  — .  Now  the  sex-limited  characters  in 
Drosophila  seem  to  be  bound  up  with  the  X 
structure,  not  repelled  by  it,  as  is  barring  in 
fowls.  Accordingly,  a  sex-limited  character 
may  be  represented  twice  in  the  female  Droso- 
phila, but  only  once  in  the  male;  or  in  other 

175 


HEREDITY 

words,  the  female  may  be  homozygous  as  re- 
gards a  sex-limited  character,  but  the  male  can 
only  be  heterozygous  (see  Fig.  53). 

Drosophila  normally  has  red  eyes,  but  the 
redness  of  the  eye  is  a  distinct  unit-character, 
sex-limited  in  heredity.  Further  males  are 
regularly  heterozygous  in  this  character,  while 
females  are  homozygous.  For  Morgan  has  ob- 
tained a  race  in  which  the  eyes  are  white, 
owing  to  the  loss  of  the  red  character;  and 
reciprocal  crosses  of  this  race  with  ordinary 
red-eyed  animals  yield  different  results.  The 
red-eyed  female  crossed  with  a  white-eyed  male 
produces  only  red-eyed  offspring,  but  the  red- 
eyed  male  crossed  with  a  white-eyed  female 
produces  offspring  only  half  of  which  are  red- 
eyed,  viz.  the  females,  whereas  the  males  are 
white-eyed. 

These  different  results  in  the  two  cases  ap- 
parently come  about  as  follows: 

First  case. 

Gametes  of   red-eyed  female  =  X-R  and  X-R, 
Gametes  of  white-eyed  male      =  X      and        — , 
Zygotes=  XX-R  (red-eyed  female),  and  =  — -X-R 
(red-eyed  male). 

176 


HEEEDITY   AND   SEX 

Second  case. 

Gametes  of  white-eyed  female  =  X  and  X, 
Gametes  of  red-eyed  male  =  X-R  and  — , 
Zygotes   =  X-X-R    (red-eyed  female),    and    -'X 
(white-eyed  male). 

A  short  condition  of  the  wings  in  Drosophila, 
which  renders  the  animal  incapable  of  flight, 
is  likewise  sex-limited  in  heredity,  as  has  been 
shown  by  Morgan.  By  crossing  two  races  of 
Drosophila,  each  of  which  possessed  a  different 
sex-limited  character,  Morgan  has  been  able  to 
combine  the  two  characters  in  a  single  race. 
Thus  was  obtained  a  race  both  white-eyed  and 
short-winged.  The  synthesis  cannot  be  made 
originally  in  a  male  individual,  but  only  in  a 
female.  For  only  in  the  female  can  the  two 
characters  be  brought  together,  each  associated 
with  a  different  X,  since  in  the  male  only  one 
X  is  present.  Although  each  sex-limited  char- 
acter seems  to  be  attached  to  or  bound  up  with 
an  X  structure,  it  evidently  has  a  material  basis 
distinct  from  X.  Otherwise  it  would  not  be 
possible  for  the  character  to  leave  one  X  and 
attach  itself  to  the  other,  as  apparently  takes 
place  in  the  female  when  the  combination  of 

177 


HEREDITY 

two  sex-limited  characters  in  the  same  gamete 
is  secured  through  a  cross.  The  combination 
is  apparently  secured  in  this  way: 

Gametes  uniting,  X-R  and  X— L, 

Zygote  formed,  X-R  X-L, 

Its  gametes,  X-R  and  X-L,  or  X-R-L  and  X. 

One  of  the  uniting  gametes,  X-E,  is  formed 
by  the  red-eyed,  short-winged  parent ;  the  other, 
X-L,  is  formed  by  the  long-winged,  white- 
eyed  parent.  The  zygote  resulting  is  a  red- 
eyed  individual,  since  it  contains  R ;  it  is  long- 
winged,  since  it  contains  L ;  it  is  a  female,  since 
it  contains  two  Xs.  Now,  its  gametes  are  of 
four  sorts,  as  indicated.  The  first  two  sorts 
result  from  simple  separation  of  the  two  Xs, 
each  with  its  associated  character,  R  in  one 
case,  L  in  the  other.  But  the  third  sort  could 
result  only  from  the  attachment  of  R  and  L 
to  the  same  X,  leaving  the  other  X  without 
either  R  or  L  as  the  fourth  kind  of  gamete. 
This  kind,  which  transmits  neither  red  eyes  nor 
long  wings,  would  represent  the  new  gametic 
combination, — white-eyed  and  with  short  wings. 

The  experimental  evidence  that  gametes  of 
178 


HEEEDITY   AND   SEX 

these  four  sorts  are  formed  by  females  of  the 
origin  described  is  as  follows :  —  When  such  a 
female  is  mated  with  a  long- winged,  white-eyed 
male,  there  are  obtained  female  offspring,  all 
of  which  are  long- winged,  but  half  of  them  are 
red-eyed,  half  white-eyed.  The  male  offspring, 
however,  are  of  four  sorts,  viz.  red  short,  white 
long,  red  long,  and  white  short.  This  result 
harmonizes  with  the  hypothesis  advanced.  For 
if  the  gametes  of  the  female  are  X-R,  X-L, 
X-R-L,  and  X,  and  those  of  the  male  are  X-L 
and  — ,  then  the  following  combinations  should 
result : 

X-L1  X-R,  red  long  female, 
X-L-  X-L,  white  long  female 
X-L-  X-R-L,  red  long  female, 
X-L-  X        ,  white  long  female, 

•  X-R,  red  short  male, 

— •  X-L,  white  long  male, 

— -  X-R-L,  red  long  male, 
•  X         ,  white  short  male. 

This  expected  result  accords  with  that  ac- 
tually obtained  by  Morgan. 

Color-blindness  in  man  is  a  sex-limited  char- 
acter, the  inheritance  of  which  resembles  that 

179 


HEREDITY 

of  white  eyes  or  short  wings  in  Drosophila, 
rather  than  of  barring  in  poultry. 

Color-blindness  is  much  commoner  in  men 
than  in  women.  A  color-blind  man,  however, 
does  not  transmit  color-blindness  to  his  sons, 
but  only  to  his  daughters,  the  daughters,  how- 
ever, are  themselves  normal  provided  the 
mother  was;  yet  they  transmit  color-blindness 
to  half  their  sons.  A  color-blind  daughter 
could  be  produced,  apparently,  only  by  the 
marriage  of  a  color-blind  man  with  a  woman 
who  transmitted  color-blindness,  since  the 
daughter  to  be  color-blind  must  have  received 
the  character  from  both  parents,  whereas  the 
color-blind  son  receives  the  character  only 
from  his  mother. 

Color-blindness  is  apparently  due  to  a  defect 
in  the  germ-cell,  —  absence  of  something  nor- 
mally  associated   there   with   an   X-structure, 
which  is  represented  twice  in  woman,  once  ii 
man.      Color-blindness    follows,    therefore, 
transmission  the  scheme  shown  in  Fig  53. 

If,  as  has  been  suggested,  the  determination 
of  sex  in  general  depends  upon  the  inheritan< 
of  a  Mendelian  factor  differentiating  the  sexe* 

180 


HEREDITY   AND    SEX 

it  is  highly  improbable  that  the  breeder  will 
ever  be  able  to  control  sex.  Male  and  female 
zygotes  should  forever  continue  to  be  produced 
in  approximate  equality,  and  consistent  inequal- 
ity of  male  and  female  births  could  result  only 
from  greater  mortality  on  the  part  of  one  sort 
of  zygote  than  of  the  other.  Only  in  partheno- 
genesis can  man  at  will  control  sex,  and  until 
he  can  produce  artificial  parthenogenesis  in  the 
higher  animals,  he  can  scarcely  hope  to  con- 
trol sex  in  such  animals. 

Negative  as  are  the  results  of  our  study  of  sex- 
control,  they  are  perhaps  not  wholly  without 
practical  value.  It  is  something  to  know  our 
limitations.  We  may  thus  save  time  from 
useless  attempts  at  controlling  what  is  uncon- 
trollable and  devote  it  to  more  profitable  em- 
ployments. 

BIBLIOGRAPHY 

BATESON,  W. 

1909.      (See  Bibliography  to  Chapter  IV.) 
CASTLE,  W.  E. 

1909.     "A  Mendelian  View  of   Sex-heredity."    Science, 

N.  S.,  vol.  29,  pp.  395-400. 
CU&NOT,  L. 

1900.    "Sur  la  determination  du  sexe  chez  les  animaux." 
Bull.  Sci.  de  la  France  et  de  la  Belgique, 

13  181 


HEREDITY 

MOEGAN,   T.   H. 

1909.  "A  Biological  and  Cytological  Study  of  Sex  Deter- 
mination  in   Phylloxerans   and  Aphids."     Journal  of 
Experimental  Zoology,  7,  pp.  239-352. 

1910.  "Sex-limited  Inheritance  in  Drosophila."    Science, 
N.  S.t  32,  pp.  120-122. 

1911.  "The  Application  of  the  Conception  of  Pure  Lines 
to  Sex-limited  Inheritance  and  to  Sexual  Dimorphism." 
The  American  Naturalist,  45,  pp.  65-78. 

Russo,  A. 

1909.     "Studien  iiber  die    Bestimmung   des   weiblichen 

Geschlectes."    G.  Fischer,  Jena. 
WILSON,  E.  B. 

1909.  "Recent  Researches  on  the  Determination  and 
Heredity  of  Sex."    Science,  N.  S.,  29,  pp.  53-70. 

1910.  "The  Chromosomes  in  Relation  to  the  Determina- 
tion of  Sex."    Science  Progress,  5,  pp.  570-592. 

For  references  to  the  earlier  literature   see   CUENOT   and 
BATESON. 


INDEX 


Abraxas,  sex  determination  in, 

174. 
Atavism,  62. 

von  Baehr,  165. 
Basile,  159. 
Bateson,  37,  110,  174. 
Baur,  61. 
Bos,  147. 
Buttercup,  107. 

Cattle,  polled,  39,  102. 

Ciona,  146. 

Color  blindness,  179. 

Correns,  34. 

Coutagne,  99. 

Cross-breeding,  effects  of,  on 

vigor,  149. 
Cuenot,  158. 

Daphnia,  116,  164. 
Darwin,  7,  62,  106. 
Davenport,  100. 
Donraster,  174. 
Drosophila,  146. 
sex   limited   inheritance   in, 
174. 


East,  130,  138. 


Egg,  fertilization  of,  11. 
of  Nereis,  12. 
of  mouse,  13. 
of  sea-urchin,  18. 

Factors,  inhibiting,  55. 

multiple,  62,  131. 
Farabee,  39. 
Fern,  20. 

prothallus  of,  22. 
spores  of,  22. 
Fingers,  inheritance  of  short, 

39. 

Fixation  of  a  reversionary  char- 
acter, 68. 

Fowls,  Andalusian,  55. 
crosses  of,  53. 

sex  limited   inheritance  in, 
170. 

Gamete,  definition  of,  25. 
Guinea-pig,  angora,  38,  42,  47. 

black  crossed  with  white,  34. 

new  variety  of,  84. 

polydactylous,  100,  121. 

reversion  in,  63. 

rough  crossed  with  smooth, 

38,  41,  47,  98. 
Hare,  Belgian,  30. 


183 


INDEX 


Heape,  30. 

Heredity,  collateral,  6. 

definition  of,  6. 
Hyalodaphnia,  117. 

Jennings,  111. 
Johannsen,  45,  106. 

Little,  59. 

Maize,  ear-length  in,  138. 
Maturation  of  egg,  15,  20. 

of  sperm-cells,  17. 
Mendel,  7,  34. 

Mendelian  ratios,  35,  45,  50,  59. 
Mice,  pale-colored,  81. 

pink-eyed,  79. 

spotted,  83. 

yellow,  57. 

Morgan,  91,  165,  174. 
Mus  alexandrinus,  91. 
Mus  norvegicus,  94. 
Mus  rattus,  91,  94. 
Mutilations,  inheritance  of,  29. 

Nilsson-Ehle,  131. 

Parthenogenesis,  artificial,  18. 

sex  determination  in,  162. 
Pigeons,  reversion  in,  62. 
Phenotype,  45. 
Phillips,  31. 
Prepotency,  101. 
Punnett,  159. 


Rats,  91. 

inbreeding  in,  147. 

selection  in,  123. 
Reduction,  in  fern,  22. 
Reversion,  62. 
Riddle,  87. 
Riley,  158. 

Rodents,  coloration  of,  72. 
Rotifers,  sex  determination  in, 

164. 
Russo,  159. 

Schenck,  159. 
Self-fertilization,  145. 
Sheep,  horns  in,  102. 
Silk  moths,  99. 
Snapdragon,  golden  variety  of, 

61. 
Spermatogenesis,  17. 

of  wasp,  24. 

Squash  bug,  sex  determination 
in,  168. 

Tornier,  102. 
Transplantation  of  egg,  30. 

of  ovary,  31. 
Treat,  158. 

Unit-characters,  38. 
De  Vries,  34,  89,  106. 

Weismann,  29,  148. 
Wilson,  168. 
Woltereck,  117. 


Rabbits,    size   inheritance   in, 
129. 


Zygote,  definition  of,  25. 
184 

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This  work  is  the  result  of  many  years  of  study  and  teaching.  Tt  is 
the  first  attempt  in  any  language  to  bring  together  all  the  best  that  has 
been  ascertained  about  the  critical  period  of  life  which  begins  with 
puberty  in  the  early  teens  and  ends  with  maturity  in  the  middle  twenties, 
and  it  is  made  by  the  one  man  whose  experience  and  ability  pre-emi- 
nently qualify  him  for  such  a  task.  The  work  includes  a  summary  of 
the  author's  conclusions  after  twenty-five  years  of  teaching  and  study 
upon  some  of  the  most  important  themes  in  Philosophy,  Psychology, 
Religion,  and  Education. 

The  nature  of  the  adolescent  period  is  the  best  guide  to  education 
from  the  upper  grades  of  the  grammar  school  through  the  high  school 
and  college.  Throughout,  the  statement  of  scientific  facts  is  followed 
systematically  by  a  consideration  of  their  application  to  education,  pe- 
nology, and  other  phases  of  life. 

Juvenile  diseases  and  crime  have  each  special  chapters.  The  changes 
of  each  sense  during  this  period  are  taken  up.  The  study  of  normal 
psychic  life  is  introduced  by  a  chapter  describing  both  typical  and  excep- 
tional adolescents,  drawn  from  biography,  literature,  lives  of  the  saints, 
and  other  sources. 

The  practical  applicauons  of  some  of  the  conclusions  of  the  scientific 
part  are  found  in  separate  chapters  on  the  education  of  girls,  coeduca- 
tion and  it£>  relations  to  marriage,  fecundity,  and  family  life,  as  seen  by 
statistics  in  American  colleges,  with  a  sketch  of  an  ideal  education  for 
girls. 

Another  chapter  treats  with  some  detail  and  criticism  the  various 
kinds  and  types  of  organization  for  adolescents  from  plays  and  games  to 
the  Y.  M.  C.  A.,  Epworth  League,  and  other  associations  devised  for 
the  young. 

The  problem  of  the  High  School,  its  chief  topics  and  methods,  is 
considered  from  the  standpoint  of  adolescence,  and  some  very  important 
modifications  are  urged.  It  closes  with  the  general  consideration  of  the 
relations  of  a  higher  to  a  lower  civilization  from  this  standpoint. 

D.     APPLETON     AND     COMPANY,     NEW     YORK. 


DATE  DUE  SLIP 

UNIVERSITY  OF  CALIFORNIA  MEDICAL  SCHOOL  LIBRARY 

THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


1935 


lm-4,'34 


